added return statement for simplify_product_function

src/main/py/read_math_function.py

@@ -105,3 +105,4 @@ def simplify_product_function(type, function):
         term_coefficient, term_variables = simplified_term
         result_coefficient *= term_coefficient
         result_variables += term_variables
+        return [f"{result_variables}^{len(terms)}"]