revset: parse x^:: as (x^):: (issue5764) We have to make '::' a valid primary expression to parse 'x^::' as '(x)^(::)' first, but that doesn't change the language because a prefix operator '::y' precedes a primary '::'. I can't think of an intuitive meaning of '::', so it's just rejected. Given 'x::y' can be considered to default to {x = roots(), y = heads()}, '::' could be 'roots()::heads()', which seems not any useful.