  Bestvina and Feighn showed that a morphism S --> T between two simplicial
trees that commutes with the action of a group G can be written as a product of
elementary folding operations. Here a more general morphism between simplicial
trees is considered, which allow different groups to act on S and T. It is
shown that these morphisms can again be written as a product of elementary
operations: the Bestvina-Feighn folds plus the so-called `vertex morphisms'.
Applications of this theory are presented. Limits of infinite folding sequences
are considered. One application is that a finitely generated inaccessible group
must contain an infinite torsion subgroup.
