  Implementing a complex concept as an executable model in a strongly typed,
purely functional language hits a sweet spot between mere simulation and formal
specification. For research and education it is often desirable to enrich the
algorithmic code with meta-logical annotations, variously embodied as
assertions, theorems or test cases. Checking frameworks use the inherent
logical power of the functional paradigm to approximate theorem proving by
heuristic testing. Here we propose several novel idioms to enhance the
practical expressivity of checking, namely meta-language marking, nominal
axiomatics, and constructive existentials. All of these are formulated in
literate Haskell'98 with some common language extensions. Their use and impact
are illustrated by application to a realistic modeling problem.
