bye-bye, by by (change "by by" to "by")
bye-bye, by by (change "by by" to "by")

Fix several instances of "by by".

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm