exp ::= x                           -- variable
      | exp exp                     -- application
      | exp `x` exp                 -- infix application
      | exp op exp                  -- infix operation
      | fun x0...xn -> exp          -- anonymous function
      | let x = exp in exp          -- let binding
      | if exp then exp else exp    -- if expression
      | exp, exp                    -- tuple
      | i                           -- int literal
      | b                           -- bool literal
      | exp :: ty                   -- type annotation

ty  ::= a0...an
      | int
      | bool
      | unit
      | ty -> ty
      | ty * ty