add FreeMagma as an example of a magma

[mwageringel]

This ticket adds an example of a magma.

sage: M = Magmas().example(); M
An example of a magma: the free magma generated by ('a', 'b', 'c', 'd')
sage: a, b, c, d = M.magma_generators()
sage: a * (b * c) * (d * a * b)
'((a*(b*c))*((d*a)*b))'

This is in part motivated by this ask-sagemath question and by the seeming lack of magmas in Sage.

Component: categories

Author: Markus Wageringel

Branch/Commit: cad7678

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/29529

[mwageringel]

Author: Markus Wageringel

[mwageringel]

Branch: u/gh-mwageringel/29529

[mwageringel]

New commits:

cad7678

29529: add FreeMagma as an example of a magma

[mwageringel]

Commit: cad7678

[tscrim]

Reviewer: Travis Scrimshaw

[tscrim]

comment:2

LGTM.

Feel free to cc me on tickets like this too.

[mwageringel]

comment:3

Thank you.

[vbraun]

Changed branch from u/gh-mwageringel/29529 to cad7678