tree <-> polish transformations

[Guido.Bosch@loria.fr (Guido Bosch)]

I need a functions that transformes a given VTP tree in a string that
corresponds  to its polish representation, and vice versa:


	tree-to-polish-string : VTP-TREE -> STRING
	polish-string-to-tree : STRING -> VTP-TREE


I know the two functions `{tree}:save' and `{tree}:restore' that do
sth. similar, but they write to and read from files. This is not
exactly what I want. 

Any help would be apreciated.

Thanks,
	Guido