diff --git a/matchpy/expressions/functions.py b/matchpy/expressions/functions.py
index 36b5385..8c7faad 100644
--- a/matchpy/expressions/functions.py
+++ b/matchpy/expressions/functions.py
@@ -13,6 +13,8 @@
 
 def is_constant(expression):
     """Check if the given expression is constant, i.e. it does not contain Wildcards."""
+    if isinstance(expression, Expression):
+        return expression.is_constant
     if isinstance(expression, Operation):
         return all(is_constant(o) for o in op_iter(expression))
     return not isinstance(expression, Wildcard)
@@ -23,6 +25,8 @@ def is_syntactic(expression):
     Check if the given expression is syntactic, i.e. it does not contain sequence wildcards or
     associative/commutative operations.
     """
+    if isinstance(expression, Expression):
+        return expression.is_syntactic
     if isinstance(expression, (AssociativeOperation, CommutativeOperation)):
         return False
     if isinstance(expression, Operation):
diff --git a/matchpy/expressions/substitution.py b/matchpy/expressions/substitution.py
index f0a0800..7757909 100644
--- a/matchpy/expressions/substitution.py
+++ b/matchpy/expressions/substitution.py
@@ -23,7 +23,7 @@
 VariableReplacement = Union[Tuple['expressions.Expression', ...], Multiset, 'expressions.Expression']
 
 
-class Substitution(Dict[str, VariableReplacement]):
+class Substitution(dict):
     """Special :class:`dict` for substitutions with nicer formatting.
 
     The key is a variable's name and the value the replacement for it.