setsEqual function
Checks Sets a and b for equality.
Returns true if a and b are both null, or they are the same length and
every element in b exists in a.
Implementation
bool setsEqual(Set? a, Set? b) {
if (a == b) return true;
if (a == null || b == null) return false;
if (a.length != b.length) return false;
return a.containsAll(b);
}