 A nontrivial freely reduced word "u" in "F"("X") is called a "piece" with respect to (∗) if there exist two distinct elements "r"1, "r"2 in "R" that have "u" as maximal common initial segment.Note that if formula_2 is a group presentation where the set of defining relators "S" is not symmetrized, we can always take the "symmetrized closure" "R" of "S", where "R" consists of all cyclic permutations of elements of "S" and "S"−1.Then "R" is symmetrized and formula_3 is also a presentation of "G".