Fetch the parent of a parse tree node if one exists. Only the leftmost parse tree node in a list has a parent.
returnedPnode = pnup(aPnode)
returnedPnode - type: pnode
aPnode - type: pnode
Pnup is a primitive that does not fetch the parent. (An NLP++ pnparent function is called for or can be written by the user.)
pndown, pnsingletdown, pnprev, pnnext, Parse Tree Functions