Improve comments.

demos/system-F-type/parser.mly

@@ -33,7 +33,8 @@
 
 (* A top level is a term. See that only terms are accepted as top level
    expression. Type term like type application is not allowed.
-   Parenthesis is not mandatory for the top level expressions.
+   Parentheses are not mandatory for the top level expressions.
+   A top level expression must be ended with ;; to evaluate it (as in OCaml).
 *)
 top_level:
 | t = term ; SEMICOLON ; SEMICOLON { t }
@@ -51,6 +52,9 @@ term:
   t = term {
     F.TeAbs(bv, typevar, t)
   }
+(* Build an abstraction. For ease of reading when the variable type is long,
+  parentheses are added around the variable and the type.
+*)
 | ABSTRACTION ;
   LPARENT ;
   bv = VAR ;
@@ -81,14 +85,11 @@ term:
   t = term {
     F.TeProj(i, t)
   }
-(* Build a term application. It must be surround by parenthesis: it avoids
-   conflicts between (id [A] x), (id [B] y) and id [A] (x, id [B] y)
-   By a previous rule, we can add as many parenthesis as we want.
+(* Build a term application. No parentheses are mandatory.
  *)
-| 
+|
   t1 = term ;
   t2 = term {
- 
     F.TeApp(t1, t2)
   }
 (* Build a Let definition *)
@@ -108,7 +109,7 @@ term:
     F.TeTyAbs(id, t)
   }
 (* Surround term with nested parentheses. Allow to add an infinite number of
-   parenthesis without modifying the meaning
+   parentheses.
 *)
 | LPARENT ;
   t = term ;