feat: expression

samples/saccharine.txt

@@ -1,9 +1,9 @@
 0 := \f x.x
-inc n := \f x.f (n f x)
-add n m := m inc n
-mult n m := m (n f)
-exp m n := m n
+(inc n) := \f x.(f (n f x))
+(add n m) := (m inc n)
+(mult n m) := (m (n f))
+(exp m n) := (m n)
 
 # This is the final output.
 5 := (inc (inc (inc (inc (inc 0)))))
-exp 5 5
+(exp 5 5)