set method
create an link between 2 nodes with value
Optional tags
for setting tags
Implementation
void set(a, b, key, val, {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.setToV(t, key, val))
.mutual((f, t) => f.setValTag(t, key, tags));
}