On mappings of terms determined by hypersubstitutions
  The extensions of hypersubstitutions are mappings on the set of all terms. In the present paper we characterize all hypersubstitutions which provide bijections on the set of all terms. The set of all such hypersubstitutions forms a monoid. On the other hand, one can modify each hypersubstitution to any mapping on the set of terms. For this we can consider mappings from the set of all hypersubstitutions into the set of all mappings on the set of all terms. 
Jorg Koppitz, Slavcho Shtrakov