25.12.3 Nulled and expected symbol instances

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