Internal monoids and groups in the category of commutative cancellative   medial magmas
  This article considers the category of commutative medial magmas with cancellation, a structure that generalizes midpoint algebras and commutative semigroups with cancellation. In this category each object admits at most one internal monoid structure for any given unit. Conditions for the existence of internal monoids and internal groups, as well as conditions under which an internal reflexive relation is a congruence, are studied. 
Jorge P. Fatelo and Nelson Martins-Ferreira
math.CT