Fixtype of Syntheto function headers.
This is a product type introduced by fty::defprod.
A function header consists of the name of the function, its inputs (a list of typed variables), and its outputs (a list of typed variables.
It is useful to introduce this notion because in Syntheto not all functions are defined; some are specified, sometimes intentionally under-specified, and the goal of the synthesis process is to generate definitions that provably meet the specifications.