add method
Implementation
void add(String a, String b, bool areMutuallyExclusive) {
final key1 = a.compareTo(b) < 0 ? a : b;
final key2 = a.compareTo(b) < 0 ? b : a;
final map = this._data[key1];
if (map == null) {
this._data[key1] = {key2: areMutuallyExclusive};
} else {
map[key2] = areMutuallyExclusive;
}
}