Before this commit, the command word was highlighted as "unknown-token"
not because alias loops are invalid, as a comment incorrectly claimed,
but because the command word «a» resolved to a «b» that was ineligible
for being expanded as an alias, and there was no function/builtin/etc.
called "b".
Add a function "b" to demonstrate that alias loops are valid. I've also
filed issue #695 about the overloading of "unknown-token".
