please implement query-exchange

[Dan Jacobson]

Maybe there should be a query-exchange function, e.g.  I want to make
all the x's become y, and all the y's become x, in
atan(x()-$xc,y()-$yc). It seems one would have a hard time doing that
with existing emacs tools.  One would have to query-replace to make x
into a temp string, then do y->x, then do temp->y: to tough for the
average user.  Therefor please implement query-exchange: looks and
feels like query-replace, but does exchanges...
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