- make-lerfu-porsi-pabalvi-valsi porsi nilclaprocedure
- the parse position to advance.
- The number of characters the position will be advanced by.
Create a lerfu-porsi from an existing lerfu-porsi by advancing the index zva by the length of lefpoi.
It is an error if porsi is not of type lerfu-porsi.
Behavior is undefined if this procedure is called with a lefpoi whose length is greater than the number of characters in remaining in the parse input.