Previously 'set -ql' would only look for variables in the
immediate local scope. This was not very useful. It's also
arguably surprising, since a 'set -l' in a function, followed
by a 'set -ql' in a child block, would fail. There was also no
way to check for a function-scoped variable, since omitting the
scope would also pull in global variables.
We could revisit this and introduce an explicit function scope.