- make-lerfu-porsi-pabalvi-lerfu porsiprocedure
- the parse position to advance.
Create a lerfu-porsi from an existing lerfu-porsi by advancing the index zva by a single character.
It is an error if porsi is not of type lerfu-porsi.
Behavior is undefined if this procedure is called when lerfu-porsi-fanmo? returns #t for this porsi.