diff --git a/include/loki/details/pddl/action.hpp b/include/loki/details/pddl/action.hpp
index dea9e88..ebb205c 100644
--- a/include/loki/details/pddl/action.hpp
+++ b/include/loki/details/pddl/action.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <optional>
 #include <string>
@@ -45,7 +44,8 @@ class ActionImpl : public Base<ActionImpl>
                std::optional<Effect> effect);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ActionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/atom.hpp b/include/loki/details/pddl/atom.hpp
index 33b98a1..816eb8a 100644
--- a/include/loki/details/pddl/atom.hpp
+++ b/include/loki/details/pddl/atom.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -35,7 +34,8 @@ class AtomImpl : public Base<AtomImpl>
     AtomImpl(size_t index, Predicate predicate, TermList terms);
 
     // Give access to the constructor.
-    friend class UniqueFactory<AtomImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/axiom.hpp b/include/loki/details/pddl/axiom.hpp
index 88f7193..a598426 100644
--- a/include/loki/details/pddl/axiom.hpp
+++ b/include/loki/details/pddl/axiom.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -37,7 +36,8 @@ class AxiomImpl : public Base<AxiomImpl>
     AxiomImpl(size_t index, std::string derived_predicate_name, ParameterList parameters, Condition condition, size_t num_parameters_to_ground_head);
 
     // Give access to the constructor.
-    friend class UniqueFactory<AxiomImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/conditions.hpp b/include/loki/details/pddl/conditions.hpp
index e7098ad..f89e88b 100644
--- a/include/loki/details/pddl/conditions.hpp
+++ b/include/loki/details/pddl/conditions.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -36,7 +35,8 @@ class ConditionLiteralImpl : public Base<ConditionLiteralImpl>
     ConditionLiteralImpl(size_t index, Literal literal);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ConditionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -68,7 +68,8 @@ class ConditionAndImpl : public Base<ConditionAndImpl>
     ConditionAndImpl(size_t index, ConditionList conditions);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ConditionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -100,7 +101,8 @@ class ConditionOrImpl : public Base<ConditionOrImpl>
     ConditionOrImpl(size_t index, ConditionList conditions);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ConditionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     bool is_structurally_equivalent_to_impl(const ConditionOrImpl& other) const;
     size_t hash_impl() const;
@@ -134,7 +136,8 @@ class ConditionNotImpl : public Base<ConditionNotImpl>
     ConditionNotImpl(size_t index, Condition condition);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ConditionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -167,7 +170,8 @@ class ConditionImplyImpl : public Base<ConditionImplyImpl>
     ConditionImplyImpl(size_t index, Condition condition_left, Condition condition_right);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ConditionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -201,7 +205,8 @@ class ConditionExistsImpl : public Base<ConditionExistsImpl>
     ConditionExistsImpl(size_t index, ParameterList parameters, Condition condition);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ConditionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -235,7 +240,8 @@ class ConditionForallImpl : public Base<ConditionForallImpl>
     ConditionForallImpl(size_t index, ParameterList parameters, Condition condition);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ConditionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/declarations.hpp b/include/loki/details/pddl/declarations.hpp
index 05d5641..6bb0aff 100644
--- a/include/loki/details/pddl/declarations.hpp
+++ b/include/loki/details/pddl/declarations.hpp
@@ -33,6 +33,18 @@ namespace loki
 template<typename T>
 using PDDLElement = const T*;
 
+/**
+ * Utils
+ */
+
+template<typename T>
+struct ShallowHash;
+template<typename T>
+struct ShallowEqualTo;
+
+template<typename HolderType, typename Hash, typename EqualTo>
+class UniqueFactory;
+
 /**
  * Domain
  */
diff --git a/include/loki/details/pddl/domain.hpp b/include/loki/details/pddl/domain.hpp
index 0073a1e..471a3ea 100644
--- a/include/loki/details/pddl/domain.hpp
+++ b/include/loki/details/pddl/domain.hpp
@@ -20,9 +20,7 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/pddl/requirements.hpp"
 #include "loki/details/utils/filesystem.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <optional>
 #include <string>
@@ -55,7 +53,8 @@ class DomainImpl : public Base<DomainImpl>
                AxiomList axioms);
 
     // Give access to the constructor.
-    friend class UniqueFactory<DomainImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/effects.hpp b/include/loki/details/pddl/effects.hpp
index 5bb31db..13e7d78 100644
--- a/include/loki/details/pddl/effects.hpp
+++ b/include/loki/details/pddl/effects.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -47,7 +46,8 @@ class EffectLiteralImpl : public Base<EffectLiteralImpl>
     EffectLiteralImpl(size_t index, Literal literal);
 
     // Give access to the constructor.
-    friend class UniqueFactory<EffectImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -79,7 +79,8 @@ class EffectAndImpl : public Base<EffectAndImpl>
     EffectAndImpl(size_t index, EffectList effects);
 
     // Give access to the constructor.
-    friend class UniqueFactory<EffectImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -113,7 +114,8 @@ class EffectNumericImpl : public Base<EffectNumericImpl>
     EffectNumericImpl(size_t index, AssignOperatorEnum assign_operator, Function function, FunctionExpression function_expression);
 
     // Give access to the constructor.
-    friend class UniqueFactory<EffectImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -148,7 +150,8 @@ class EffectConditionalForallImpl : public Base<EffectConditionalForallImpl>
     EffectConditionalForallImpl(size_t index, ParameterList parameters, Effect effect);
 
     // Give access to the constructor.
-    friend class UniqueFactory<EffectImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -182,7 +185,8 @@ class EffectConditionalWhenImpl : public Base<EffectConditionalWhenImpl>
     EffectConditionalWhenImpl(size_t index, Condition condition, Effect effect);
 
     // Give access to the constructor.
-    friend class UniqueFactory<EffectImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/function.hpp b/include/loki/details/pddl/function.hpp
index ea3ee23..b4aba2b 100644
--- a/include/loki/details/pddl/function.hpp
+++ b/include/loki/details/pddl/function.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -35,7 +34,8 @@ class FunctionImpl : public Base<FunctionImpl>
     FunctionImpl(size_t index, FunctionSkeleton function_skeleton, TermList terms);
 
     // Give access to the constructor.
-    friend class UniqueFactory<FunctionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/function_expressions.hpp b/include/loki/details/pddl/function_expressions.hpp
index 4598d60..dab869b 100644
--- a/include/loki/details/pddl/function_expressions.hpp
+++ b/include/loki/details/pddl/function_expressions.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 #include <unordered_map>
@@ -56,7 +55,8 @@ class FunctionExpressionNumberImpl : public Base<FunctionExpressionNumberImpl>
     FunctionExpressionNumberImpl(size_t index, double number);
 
     // Give access to the constructor.
-    friend class UniqueFactory<FunctionExpressionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -93,7 +93,8 @@ class FunctionExpressionBinaryOperatorImpl : public Base<FunctionExpressionBinar
                                          FunctionExpression right_function_expression);
 
     // Give access to the constructor.
-    friend class UniqueFactory<FunctionExpressionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -128,7 +129,8 @@ class FunctionExpressionMultiOperatorImpl : public Base<FunctionExpressionMultiO
     FunctionExpressionMultiOperatorImpl(size_t index, MultiOperatorEnum multi_operator, FunctionExpressionList function_expressions);
 
     // Give access to the constructor.
-    friend class UniqueFactory<FunctionExpressionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -161,7 +163,8 @@ class FunctionExpressionMinusImpl : public Base<FunctionExpressionMinusImpl>
     FunctionExpressionMinusImpl(size_t index, FunctionExpression function_expression);
 
     // Give access to the constructor.
-    friend class UniqueFactory<FunctionExpressionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -193,7 +196,8 @@ class FunctionExpressionFunctionImpl : public Base<FunctionExpressionFunctionImp
     FunctionExpressionFunctionImpl(size_t index, Function function);
 
     // Give access to the constructor.
-    friend class UniqueFactory<FunctionExpressionImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/function_skeleton.hpp b/include/loki/details/pddl/function_skeleton.hpp
index 635de06..4eb4730 100644
--- a/include/loki/details/pddl/function_skeleton.hpp
+++ b/include/loki/details/pddl/function_skeleton.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -36,7 +35,8 @@ class FunctionSkeletonImpl : public Base<FunctionSkeletonImpl>
     FunctionSkeletonImpl(size_t index, std::string name, ParameterList parameters, Type type);
 
     // Give access to the constructor.
-    friend class UniqueFactory<FunctionSkeletonImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/literal.hpp b/include/loki/details/pddl/literal.hpp
index aaa1b5a..24aa715 100644
--- a/include/loki/details/pddl/literal.hpp
+++ b/include/loki/details/pddl/literal.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -35,7 +34,8 @@ class LiteralImpl : public Base<LiteralImpl>
     LiteralImpl(size_t index, bool is_negated, Atom atom);
 
     // Give access to the constructor.
-    friend class UniqueFactory<LiteralImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/metric.hpp b/include/loki/details/pddl/metric.hpp
index 0e0cb18..20002b0 100644
--- a/include/loki/details/pddl/metric.hpp
+++ b/include/loki/details/pddl/metric.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -44,7 +43,8 @@ class OptimizationMetricImpl : public Base<OptimizationMetricImpl>
     OptimizationMetricImpl(size_t index, OptimizationMetricEnum optimization_metric, FunctionExpression function_expression);
 
     // Give access to the constructor.
-    friend class UniqueFactory<OptimizationMetricImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/numeric_fluent.hpp b/include/loki/details/pddl/numeric_fluent.hpp
index a4712f1..9af54f1 100644
--- a/include/loki/details/pddl/numeric_fluent.hpp
+++ b/include/loki/details/pddl/numeric_fluent.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -33,7 +32,8 @@ class NumericFluentImpl : public Base<NumericFluentImpl>
     double m_number;
 
     // Give access to the constructor.
-    friend class UniqueFactory<NumericFluentImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     NumericFluentImpl(size_t index, Function function, double number);
 
diff --git a/include/loki/details/pddl/object.hpp b/include/loki/details/pddl/object.hpp
index 22510bb..d42a4be 100644
--- a/include/loki/details/pddl/object.hpp
+++ b/include/loki/details/pddl/object.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -35,7 +34,8 @@ class ObjectImpl : public Base<ObjectImpl>
     ObjectImpl(size_t index, std::string name, TypeList types = {});
 
     // Give access to the constructor.
-    friend class UniqueFactory<ObjectImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/parameter.hpp b/include/loki/details/pddl/parameter.hpp
index 5ac9a16..e35246a 100644
--- a/include/loki/details/pddl/parameter.hpp
+++ b/include/loki/details/pddl/parameter.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -36,7 +35,8 @@ class ParameterImpl : public Base<ParameterImpl>
     ParameterImpl(size_t index, Variable variable, TypeList types);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ParameterImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/predicate.hpp b/include/loki/details/pddl/predicate.hpp
index 57ed015..adf25ba 100644
--- a/include/loki/details/pddl/predicate.hpp
+++ b/include/loki/details/pddl/predicate.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -37,7 +36,8 @@ class PredicateImpl : public Base<PredicateImpl>
     PredicateImpl(size_t index, std::string name, ParameterList parameters);
 
     // Give access to the constructor.
-    friend class UniqueFactory<PredicateImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/problem.hpp b/include/loki/details/pddl/problem.hpp
index c1552f0..a7953f6 100644
--- a/include/loki/details/pddl/problem.hpp
+++ b/include/loki/details/pddl/problem.hpp
@@ -21,7 +21,6 @@
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
 #include "loki/details/utils/filesystem.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <optional>
 #include <string>
@@ -57,7 +56,8 @@ class ProblemImpl : public Base<ProblemImpl>
                 AxiomList axioms);
 
     // Give access to the constructor.
-    friend class UniqueFactory<ProblemImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/requirements.hpp b/include/loki/details/pddl/requirements.hpp
index 937d777..1b948c1 100644
--- a/include/loki/details/pddl/requirements.hpp
+++ b/include/loki/details/pddl/requirements.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <set>
 #include <string>
@@ -65,7 +64,8 @@ class RequirementsImpl : public Base<RequirementsImpl>
     RequirementsImpl(size_t index, RequirementEnumSet requirements);
 
     // Give access to the constructor.
-    friend class UniqueFactory<RequirementsImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/term.hpp b/include/loki/details/pddl/term.hpp
index 783f6de..3110de8 100644
--- a/include/loki/details/pddl/term.hpp
+++ b/include/loki/details/pddl/term.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -37,7 +36,8 @@ class TermObjectImpl : public Base<TermObjectImpl>
     TermObjectImpl(size_t index, Object object);
 
     // Give access to the constructor.
-    friend class UniqueFactory<TermImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
@@ -70,7 +70,8 @@ class TermVariableImpl : public Base<TermVariableImpl>
     TermVariableImpl(size_t index, Variable variable);
 
     // Give access to the constructor.
-    friend class UniqueFactory<TermImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/type.hpp b/include/loki/details/pddl/type.hpp
index 09c96d2..bee48cc 100644
--- a/include/loki/details/pddl/type.hpp
+++ b/include/loki/details/pddl/type.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -35,7 +34,8 @@ class TypeImpl : public Base<TypeImpl>
     TypeImpl(size_t index, std::string name, TypeList bases = {});
 
     // Give access to the constructor.
-    friend class UniqueFactory<TypeImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/include/loki/details/pddl/variable.hpp b/include/loki/details/pddl/variable.hpp
index 9851b39..c1feb34 100644
--- a/include/loki/details/pddl/variable.hpp
+++ b/include/loki/details/pddl/variable.hpp
@@ -20,7 +20,6 @@
 
 #include "loki/details/pddl/base.hpp"
 #include "loki/details/pddl/declarations.hpp"
-#include "loki/details/utils/unique_factory.hpp"
 
 #include <string>
 
@@ -34,7 +33,8 @@ class VariableImpl : public Base<VariableImpl>
     VariableImpl(size_t index, std::string name);
 
     // Give access to the constructor.
-    friend class UniqueFactory<VariableImpl>;
+    template<typename HolderType, typename Hash, typename EqualTo>
+    friend class UniqueFactory;
 
     void str_impl(std::ostream& out, const FormattingOptions& options) const;
 
diff --git a/src/pddl/action.cpp b/src/pddl/action.cpp
index 7f733a3..e97df23 100644
--- a/src/pddl/action.cpp
+++ b/src/pddl/action.cpp
@@ -22,6 +22,7 @@
 #include "loki/details/pddl/parameter.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/atom.cpp b/src/pddl/atom.cpp
index 1e45127..b732669 100644
--- a/src/pddl/atom.cpp
+++ b/src/pddl/atom.cpp
@@ -21,6 +21,7 @@
 #include "loki/details/pddl/term.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/axiom.cpp b/src/pddl/axiom.cpp
index 0fd32ac..31dbdcd 100644
--- a/src/pddl/axiom.cpp
+++ b/src/pddl/axiom.cpp
@@ -24,6 +24,7 @@
 #include "loki/details/pddl/predicate.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/conditions.cpp b/src/pddl/conditions.cpp
index 6653b24..95a1882 100644
--- a/src/pddl/conditions.cpp
+++ b/src/pddl/conditions.cpp
@@ -21,6 +21,7 @@
 #include "loki/details/pddl/parameter.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/domain.cpp b/src/pddl/domain.cpp
index 2b34f5a..083dffd 100644
--- a/src/pddl/domain.cpp
+++ b/src/pddl/domain.cpp
@@ -22,8 +22,10 @@
 #include "loki/details/pddl/function_skeleton.hpp"
 #include "loki/details/pddl/object.hpp"
 #include "loki/details/pddl/predicate.hpp"
+#include "loki/details/pddl/requirements.hpp"
 #include "loki/details/pddl/type.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <iostream>
diff --git a/src/pddl/effects.cpp b/src/pddl/effects.cpp
index 692c5d3..bacc1e2 100644
--- a/src/pddl/effects.cpp
+++ b/src/pddl/effects.cpp
@@ -24,6 +24,7 @@
 #include "loki/details/pddl/parameter.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <cassert>
diff --git a/src/pddl/function.cpp b/src/pddl/function.cpp
index ac07bd9..728d345 100644
--- a/src/pddl/function.cpp
+++ b/src/pddl/function.cpp
@@ -20,6 +20,7 @@
 #include "loki/details/pddl/function_skeleton.hpp"
 #include "loki/details/pddl/term.hpp"
 #include "loki/details/pddl/visitors.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/function_expressions.cpp b/src/pddl/function_expressions.cpp
index 3893f71..5156d8a 100644
--- a/src/pddl/function_expressions.cpp
+++ b/src/pddl/function_expressions.cpp
@@ -20,6 +20,7 @@
 #include "loki/details/pddl/function.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <cassert>
diff --git a/src/pddl/function_skeleton.cpp b/src/pddl/function_skeleton.cpp
index d4ab9d3..76ddbd9 100644
--- a/src/pddl/function_skeleton.cpp
+++ b/src/pddl/function_skeleton.cpp
@@ -19,6 +19,7 @@
 
 #include "loki/details/pddl/parameter.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/literal.cpp b/src/pddl/literal.cpp
index df4be32..ab4ef08 100644
--- a/src/pddl/literal.cpp
+++ b/src/pddl/literal.cpp
@@ -19,6 +19,7 @@
 
 #include "loki/details/pddl/atom.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/metric.cpp b/src/pddl/metric.cpp
index e18c21f..7e89ea3 100644
--- a/src/pddl/metric.cpp
+++ b/src/pddl/metric.cpp
@@ -20,6 +20,7 @@
 #include "loki/details/pddl/function_expressions.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <cassert>
diff --git a/src/pddl/numeric_fluent.cpp b/src/pddl/numeric_fluent.cpp
index 1022202..058f986 100644
--- a/src/pddl/numeric_fluent.cpp
+++ b/src/pddl/numeric_fluent.cpp
@@ -18,6 +18,7 @@
 #include "loki/details/pddl/numeric_fluent.hpp"
 
 #include "loki/details/pddl/function.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/object.cpp b/src/pddl/object.cpp
index fb0b7a5..2997c7a 100644
--- a/src/pddl/object.cpp
+++ b/src/pddl/object.cpp
@@ -19,6 +19,7 @@
 
 #include "loki/details/pddl/type.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/parameter.cpp b/src/pddl/parameter.cpp
index b592bf9..d06c68c 100644
--- a/src/pddl/parameter.cpp
+++ b/src/pddl/parameter.cpp
@@ -20,6 +20,7 @@
 #include "loki/details/pddl/type.hpp"
 #include "loki/details/pddl/variable.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <cassert>
diff --git a/src/pddl/predicate.cpp b/src/pddl/predicate.cpp
index c2367e5..22c664c 100644
--- a/src/pddl/predicate.cpp
+++ b/src/pddl/predicate.cpp
@@ -21,6 +21,7 @@
 #include "loki/details/pddl/type.hpp"
 #include "loki/details/pddl/variable.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <memory>
diff --git a/src/pddl/problem.cpp b/src/pddl/problem.cpp
index c33d77a..72de3e5 100644
--- a/src/pddl/problem.cpp
+++ b/src/pddl/problem.cpp
@@ -29,6 +29,7 @@
 #include "loki/details/pddl/type.hpp"
 #include "loki/details/pddl/visitors.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <iostream>
diff --git a/src/pddl/requirements.cpp b/src/pddl/requirements.cpp
index a3da2ea..9f94b26 100644
--- a/src/pddl/requirements.cpp
+++ b/src/pddl/requirements.cpp
@@ -17,6 +17,7 @@
 
 #include "loki/details/pddl/requirements.hpp"
 
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 #include <cassert>
diff --git a/src/pddl/term.cpp b/src/pddl/term.cpp
index 8c88e0e..577a396 100644
--- a/src/pddl/term.cpp
+++ b/src/pddl/term.cpp
@@ -20,6 +20,7 @@
 #include "loki/details/pddl/object.hpp"
 #include "loki/details/pddl/variable.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/type.cpp b/src/pddl/type.cpp
index 728f08e..d769117 100644
--- a/src/pddl/type.cpp
+++ b/src/pddl/type.cpp
@@ -18,6 +18,7 @@
 #include "loki/details/pddl/type.hpp"
 
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki
diff --git a/src/pddl/variable.cpp b/src/pddl/variable.cpp
index abc9adb..88d2977 100644
--- a/src/pddl/variable.cpp
+++ b/src/pddl/variable.cpp
@@ -23,6 +23,7 @@
 #include "loki/details/pddl/parameter.hpp"
 #include "loki/details/pddl/term.hpp"
 #include "loki/details/utils/collections.hpp"
+#include "loki/details/utils/equal_to.hpp"
 #include "loki/details/utils/hash.hpp"
 
 namespace loki