Fix typo
Make it possible to assert variable amount of inputs

This means you can assert that 1..n items of given type are passed to the
function. Example: .on([is.number]).
Document the fact that invariants may depend on each other better

Updated the test while at it. There was some weird recursion going on.
annotate.js probably should catch this using function meta information.
Implement a way to check postconditions

Use .satisfies(<rule>) for this purpose.
Make warning messages more informative
Bump version