lib/ir: improve documentation

src/cwe_checker_lib/src/intermediate_representation/expression.rs

@@ -233,9 +233,12 @@ impl Expression {
 
     /// Compute a recursion depth for the expression.
     ///
-    /// Because of the recursive nature of the [Expression] type,
-    /// overly complex expressions are very costly to clone, which in turn can negatively affect some analyses.
-    /// The recursion depth measure can be used to detect and handle such cases.
+    /// Equal to the height of the expression's AST.
+    ///
+    /// Because of the recursive nature of the [Expression] type, overly complex
+    /// expressions are very costly to clone, which in turn can negatively
+    /// affect some analyses. The recursion depth measure can be used to detect
+    /// and handle such cases.
     pub fn recursion_depth(&self) -> u64 {
         use Expression::*;
         match self {

src/cwe_checker_lib/src/intermediate_representation/term.rs

@@ -28,6 +28,8 @@ pub struct Tid {
 pub type ProgramTid = Tid;
 /// Identifier of a function term.
 pub type FunctionTid = Tid;
+/// Identifier of an external function term.
+pub type ExtFunctionTid = Tid;
 /// Identifier of a block term.
 pub type BlockTid = Tid;
 /// Identifier of an instruction term.