me> bar[ancestor::foo and ancester::*[self::foo or descendent::foo or
me> self::a or self::b]]

as previously mentioned that fails if there are extra foo in the
tree, but I think

bar[ancestor::*[not(self::a) or not(self::b)][1][self::foo]]

meets the spec: the first ancestor that isn't a or b is foo.

or, going the other way:

foo[descendent::*[not(self::a) or not(self::b)][1][self::bar]]

foo such that the first descendent that isn't a or b is bar.

David

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