contract: Some auxiliaries for contract checking

This package contains libraries to deal with contracts as described in this paper:

Sergio Antoy, Michael Hanus: [Contracts and Specifications for Functional Logic Programming](http://dx.doi.or g/10.1007/978-3-642-27694-1_4), Proc. of the 14th International Symposium on Practical Aspects of Declarative Languages (PADL 2012), Springer LNCS 7149, pp. 33-47, 2012