This function simplify the linking of two nodes with a link. Its definition is:
bool attach(Node *a,Node *b, Link *l) { bool ret=true; l->link_to(b);l->link_from(a); if (a) ret&=a->link_to(l); if (b) ret&=b->link_from(l); return ret; }so it can be used also without checking the validity of the node pointers. Note: the order of the operations is important.
Neuron
assumes that the link
passed to its methods has a valid node at the other end.