link method
create an link between 2 nodes
Optional tags
for setting tags
Implementation
void link(a, b, {List tags = const []}) {
_Tuple2(a, b)
.map((v) => _map_add_or_get(v, _newNode))
.effect
.mutual((f, t) => f.setFrom(t))
.mutual((f, t) => f.setTo(t))
.mutual((f, t) => f.setTag(t, tags));
}