The reader is referred to Hanna Neumann [12], [13] for notation, terminology and basic facts relating to varieties of groups. Recall that a variety of universal algebras is a class of universal algebras closed under the operations of forming subalgebras, cartesian products and quotient algebras. Equivalently a variety is the class of universal algebras satisfying a given set of identical relations (Birkoff [1]; see also Neumann [11] or Cohn [9] for varieties of universal algebras).