Judgement = Env "|-" Term ":" Type
	  | Env "|-" Term
	  | Term ":" Type
	  | Term

Term = Var
     | "fun" Var "->" Term
     | Term Term
     | "(" Term ")"

Type = Var
     | Type "->" Type
     | "(" Type ")"

Var = variable starting with a lower case letter
    | metavariable starting with an upper case letter

Env = Var
    | Var ":" Type "," Env
    | "(" Env ")"