Implement subtraction in evaluator

lib/evaluator.c

@@ -6,5 +6,12 @@ int evaluate(const expression_t *expression)
 		return expression->number;
 	const int x = evaluate(expression->application.operands[0]);
 	const int y = evaluate(expression->application.operands[1]);
-	return x + y;
+	switch (expression->application.operator) {
+	case OPERATOR_ADD:
+		return x + y;
+	case OPERATOR_SUBTRACT:
+		return x - y;
+	default:
+		return -1;
+	}
 }