diff --git a/include/Expression.hpp b/include/Expression.hpp
index b832e72..e2ecb52 100644
--- a/include/Expression.hpp
+++ b/include/Expression.hpp
@@ -19,6 +19,7 @@
 
 #include <optional>
 #include <string>
+#include <vector>
 
 namespace expression
 {
@@ -33,4 +34,6 @@
 
 std::optional<Operation> parseOperation(std::string& op);
 int evaluate(int a, Operation op, int b);
+int evaluate(int substitute, std::vector<std::string>::iterator& curr,
+             std::vector<std::string>::iterator&& end);
 } // namespace expression