fixup! refactor: use clock from SciMLBase, fix tests

src/discretedomain.jl

@@ -183,18 +183,18 @@ julia> x(k)      # no shift
 x(t)
 
 julia> x(k+1)    # shift
-Shift(t, 1)(x(t))
+Shift(1)(x(t))
 ```
 """
 struct ShiftIndex
     clock::Union{InferredTimeDomain, TimeDomain, IntegerSequence}
     steps::Int
     function ShiftIndex(
-            clock::Union{TimeDomain, InferredTimeDomain} = Inferred, steps::Int = 0)
+            clock::Union{TimeDomain, InferredTimeDomain, IntegerSequence} = Inferred, steps::Int = 0)
         new(clock, steps)
     end
     ShiftIndex(dt::Real, steps::Int = 0) = new(Clock(dt), steps)
-    ShiftIndex(t::Num, steps::Int = 0) = new(IntegerSequence(), steps)
+    ShiftIndex(::Num, steps::Int) = new(IntegerSequence(), steps)
 end
 
 function (xn::Num)(k::ShiftIndex)
@@ -207,11 +207,6 @@ function (xn::Num)(k::ShiftIndex)
     args = Symbolics.arguments(vars[]) # args should be one element vector with the t in x(t)
     length(args) == 1 ||
         error("Cannot shift an expression with multiple independent variables $x.")
-    t = args[]
-    if hasfield(typeof(clock), :t)
-        isequal(t, clock.t) ||
-            error("Independent variable of $xn is not the same as that of the ShiftIndex $(k.t)")
-    end
 
     # d, _ = propagate_time_domain(xn)
     # if d != clock # this is only required if the variable has another clock