Examples and counterexamples for magmas, semigroups and monoids
Examples and counterexamples for magmas, semigroups and monoids

See also issue #1457:
* c0g, cgsu, df-0g, df-gsum moved back (because of mmj2 soundness check)
* auxiliary theorems moved from AV's mathbox into main set.mm
* Subsection "Examples and counterexamples for magmas, semigroups and monoids" moved from AV's mathbox into main set.mm

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