Signature Assertions
Let F denote the abstract set of field names and M, method names.
A field signature assertion is a term n.f:t, n?N, f? F, t? SigTy.
A method signature assertion n.f:s, n ? N, m?M. s a method signature of the form t*? r. t* is a sequence from SigTy, r ? SigTy ? {void}.
A class assertion is a term n:class or n:interface.
A signature assertion is either a field signature assertion, a method signature assertion, or a class assertion.