Thank you for your help very much. Glad to help. Now, let me go back to my sort constraints. More substantial changes are a strict refining of its sort, adding a feature, or having a subterm change. I am afraid that you have not understood the essence of psi-terms.
Unification affects the sort of a variable by intersecting it with another one's.
Another strict refinement may be measured as not the root, but one of the subterms being refined. As a matter of fact, I don't know why it was introduced. This has been done for languages over selective domains. Work on the interpreter was stopped at PRL in favor of working out the compiler. And recently, even the latter has been stopped.