implement meta token replacement

example.mlc

@@ -1,3 +1,4 @@
+#with mathlib.mlc
 variable:=-3; c := (a+b- 3) * 23 + variable; d := c - a;Natural : Number (n) := {n >= 0};faculty : Natural (n) -> Natural := if n = 0 then 1 else faculty (n-1) * n end;
 String Natural (n) := {Character * n};hello_word -> String := "Hello World!";
 first_letter -> Character := 'a';