Experiment: a stumbling block that might force a rethink

function calls are all wrapped in a transaction implicitly, so there is *no way* for a single
function call to speak to other functions, dashing my immediate plan for the replicate() API.
There might be a way for other functions to speak to replicate(), however, and if so that might be enough.
Experiment on the semantics of RETURNS SETOF + python generators.