Re: query-replace-interactive not documented

[Richard Stallman]

    > I left out the part about `\\#&' and `\\#N' because I don't
    > understand what they do.  I am not sure what "respective numeric
    > values" means.  When I tried \#&, it seemed to insert the
    > replacement count followed by an &.

    Not _within_ a Lisp expression, namely something like
    \,(format "%d %d" \#& \#2).  And if you had a match like
    \([0-9]+\), then you can do arithmetic on it with something like
    \,(1+ \#1).

I have a general idea now, but, what's the difference in meaning
between \#& and \&?  Between \#N and \N?