> Having a feature that makes the language simple which is not
> used by anyone (or only rarely) does not justify complicating the
> parser more than necessary.

I wouldn't say that this is unused.  I came across this issue when
trying to write an ACL method, looking something like this:
only( a :guest and not the @user ) do end # => 'a' and 'the' are both
methods returning boolean based on supplied arg

Am I unable to have methods which rely on the result of a boolean
expression, without having to resort to double parenthesis, or
remembering to put the subtle extra space? That surprises me very
much, and definitely prevents me from using it. It's a source of hard-
to-find bugs.