Sammy

(* Pretty-printing. *)
local
  fun toStringh TRUE = "TRUE"
  |   toStringh FALSE = "FALSE"
  |   toStringh Var e = e
  |   toStringh Not e = "~" ^ noth e
  |   toStringh Or (e1, e2) = orh e1 ^ " | " ^ orh e2
  |   toStringh And (e1, e2) = andh e1 ^ " & " ^ andh e2
  |   toStringh Implies (e1, e2) = toStringh e1 ^ " -> " ^ toStringh e2

  and noth Or (e1, e2) = toStringh e1 ^ toStringh e2
  |   noth And (e1, e2) = toStringh e1 ^ toStringh e2
  |   noth Implies (e1, e2) = toStringh e1 ^ toStringh e2

  and andh Or (e1, e2) = toStringh e1 ^ toStringh e2
  |   andh Implies (e1, e2) = toStringh e1 ^ toStringh e2

  and orh Implies (e1, e2) = "(" ^ toStringh e1 ^ toStringh e2 ^ ")"

in
  fun toString e = toStringh e
end