[DESTRUCTIVE]
Adds a link from the `from' node to the `to' node by inserting it at the given position. Thus, the new link is at position `pos', and its previous contents is at position `pos'+1 etc. If `pos' is less than one (1), or greater than the number of entries plus one, the function immediately returns ERR_POS (5).
If `pos' is one (1), the given group and field are created unless they exist beforehand.
Note: LINK creates a new version of `from', so therefore it binds `from', `to' may be bound or unbound. You may want to UBL_UNBINDLABEL() `to' before you LINK() to it .
Function returns: