Tweak section on Gottlob Frege's _Begriffsschrift_
Tweak section on Gottlob Frege's _Begriffsschrift_

Add improvements per mailing list discussion, e.g.,
add specific page numbers, and make it clear that
Frege had a second-order system.

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

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