contains method
Implementation
bool contains(Range<T> range) {
if (empty && range.empty) {
return true;
} else if (empty && range.notEmpty) {
return false;
} else if (notEmpty && range.empty) {
return true;
}
return _compareBounds(lowerBound, range.lowerBound) <= 0 &&
_compareBounds(upperBound, range.upperBound) >= 0;
}