/**
* Procedure testing if a ship is in (one of) the specified track bits.
* @param v Vehicle to examine.
* @param data Pointer to TrackBits.
* @return \a v if conditions are met, else \c NULL.
*/
static Vehicle *EnsureNoShipOnTrackProc(Vehicle *v, void *data)
{
if (v->type != VEH_SHIP) return NULL;
Ship *s = Ship::From(v);
TrackBits tb = *(TrackBits *)data;
if (s->state & tb) return v;
return NULL;
}