is bound in only one of the environments. This is necessary in the
case where a library uses an unbound keyword (e.g. "with" in (chibi
loop)), but you want to use it along with a binding for the
keyword (e.g. "with" in (chibi show)). The alternative to work with
the current logic is to always require such keywords to be bound, in
this case to add a dummy "with" auxiliary syntax binding to (chibi
loop), however this doesn't seem any safer than the new logic, and the
whole point of the feature is convenience. Fixes issue #221.
