Tidy up error message

pysr/sr.py

@@ -129,12 +129,12 @@ def _handle_constraints(binary_operators, unary_operators, constraints):
     for op in binary_operators:
         if op not in constraints:
             constraints[op] = (-1, -1)
-        if op in ["plus", "sub"]:
+        if op in ["plus", "sub", "+", "-"]:
             if constraints[op][0] != constraints[op][1]:
                 raise NotImplementedError(
-                    "You need equal constraints on both sides for - and *, due to simplification strategies."
+                    "You need equal constraints on both sides for - and +, due to simplification strategies."
                 )
-        elif op == "mult":
+        elif op in ["mult", "*"]:
             # Make sure the complex expression is in the left side.
             if constraints[op][0] == -1:
                 continue