Description: Amagma is a set equipped with a closed operation. Definition 1 of
BourbakiAlg1 p. 1, or definition of a groupoid in section I.1 of Bruck
p. 1. Note: The term "groupoid" is now widely used to refer to other
objects: (small) categories all of whose morphisms are invertible, or
groups with a partial function replacing the binary operation. Therefore,
we will only use the term "magma" for the present notion in set.mm.
(Contributed by AV, 6-Jan-2020)