gen

Given a spec, returns the generator for it, or throws if none can be constructed. Optionally an overrides map can be provided which should map spec names or paths (vectors of keywords) to no-arg generator-creating fns. These will be used instead of the generators at those names/paths. Note that parent generator (in the spec or overrides map) will supersede those of any subtrees. A generator for a regex op must always return a sequential collection (i.e. a generator for s/? should return either an empty sequence/vector or a sequence/vector with one item in it)