rem: return;  _d->unreg(&_p->uri, &_p->uri, (void*)callback, _p);
add: _d->unreg(&_p->uri, &_p->uri, (void*)callback, _p);
context: void remove_presentity(pdomain_t* _d, struct presentity* _p){	return;	/* FIXME: experimental */	_d->unreg(&_p->uri, &_p->uri, (void*)callback, _p);		LOG(L_ERR, "unregistering callback to %.*s, %p\n", _p->uri.len, _p->uri.s,_p);		LOG(L_WARN, "remove_presentity _p=%p p_uri=%.*s\n", _p, _p->uri.len, _p->uri.s);	slot_rem(_p->slot, _p, &_d->first, &_d->last);}