Handle nesting and subtraction in reader

lib/include/expression.h

@@ -4,10 +4,10 @@
 #include <stdbool.h>
 
 typedef enum {
-	OPERATOR_ADD,
-	OPERATOR_SUBTRACT,
-	OPERATOR_MULTIPLY,
-	OPERATOR_DIVIDE,
+	OPERATOR_ADD = 0,
+	OPERATOR_SUBTRACT = 1,
+	OPERATOR_MULTIPLY = 2,
+	OPERATOR_DIVIDE = 3,
 } operator_t;
 
 struct _expression_t;