Fixed expressions to be able to do math with nested expressions.

__write.cpp

@@ -1,6 +1,6 @@
 #include "__write.h"
 #include "Exception.h"
-#include "Expression.h"
+#include "Operand.h"
 #include <iostream>
 #include <fcntl.h>
 #include <unistd.h>