The parameter is special as it takes no annotation but a 'via' keyword followed by an expression that should unify to a Fuzzer<a>, where Fuzzer<a> = fn(Seed) -> (Seed, a). The current commit only allow name identifiers for now. Ultimately, this may allow full expressions.
This was a bit more tricky than anticipated but played out nicely in
the end. Now we have one holistic way of parsing todos and errors
instead of it being duplicated between when/clause and sequence. The
error/todo parser has been moved up to the expression part rather than
being managed when parsing sequences. Not sure what motivated that to
begin with.
Fixes#621.
KtorZ