{-# OPTIONS -fallow-undecidable-instances #-}

{- |

Module : $Header$

Description : Instance of class Logic for propositional logic

Copyright : (c) Dominik Luecke, Uni Bremen 2007

License : similar to LGPL, see HetCATS/LICENSE.txt or LIZENZ.txt

Maintainer : luecke@tzi.de

Stability : experimental

Portability : portable

Definition of signatures for propositional logic

Ref.

Till Mossakowski, Joseph Goguen, Razvan Diaconescu, Andrzej Tarlecki.

What is a Logic?.

In Jean-Yves Beziau (Ed.), Logica Universalis, pp. 113-@133. Birkhäuser.

2005.

-}

module Propositional.Sign where

import qualified Common.Lib.Set as Set

import Common.Id

-- Signatures are just sets

data Sign = Sign {items :: Set.Set Id}

deriving (Eq, Show)

-- | determines whether a signature is vaild

-- all sets are ok, so glued to true

isLegalSignature :: Sign -> Bool

isLegalSignature _ = True