This paper presents some of our requirements for a Z typechecker:  that the typechecker accept all well-typeable formulations, however  contrived; that it gather information about uses of declarations as  needed to support interactive browsing and formal reasoning; that it fit  the description given by draft standard Z; and that it be able to check  some particular extensions to Z that are intended to allow explicit definitions  of schema calculus operators. The paper presents a specification  of such a Z typechecker, which we have implemented

C.T. Sennett

I. Toyn

J.M. Spivey