Help people understand substitution notation in conventions (#2542)
Help people understand substitution notation in conventions (#2542)

The proper substitution notation is different than many others'
(for good reason). This commit adds to the conventions section
a simple way to remember how the substitution notation works,
as a way to help new readers. The text is from in the Metamath book.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>
metamath/set.mm,metamath/set.mm,metamath/set.mm