text: thanks to the convention that terms that do not include an equals sign are taken to be equal to the group identity.  Such terms are called relators, distinguishing them from the relations that do include an equals sign.
  Every group has a presentation, and in fact many different presentations; a presentation is often the most compact way of describing the structure of the group.
  A closely related but different concept is that of an absolute presentation of a group.
  Background
  A free group on a set S is a group where each element can be uniquely described as a finite length product of the form:
  where the si are elements of S, adjacent si are distinct, and ai are non-zero integers (but n may be zero). In less formal terms, the group consists of words in the generators and their inverses, subject only to canceling a generator with an adjacent occurrence of its inverse.
  If G is any group, and S is a generating subset of G, then every element of G is also of the above form; but in general, these products will not uniquely describe an element of G.
  For example, the dihedral group D8 of order sixteen can be generated by a rotation, r, of order 8; and a flip, f, of order 2; and certainly any element of D8 is a product of rs and fs.