25.12.1 Nulled and completed symbol instances

A symbol instance cannot trigger both a MARPA_EVENT_SYMBOL_NULLED and a MARPA_EVENT_SYMBOL_COMPLETED event at the same location. This is because a symbol instance must be zero-length to trigger a MARPA_EVENT_SYMBOL_NULLED event, but must be of non-zero length to trigger a MARPA_EVENT_SYMBOL_COMPLETED event. That is, if the symbol instance is sym@start-end, a MARPA_EVENT_SYMBOL_NULLED event can trigger only if start=end, while the MARPA_EVENT_SYMBOL_COMPLETED event can trigger only if start < end.