public IEnumerator <KeyValuePair <K, V> > GetEnumerator()
{
HashSet <KeyValuePair <K, V> > dupSet = new HashSet <KeyValuePair <K, V> >(
EqualityComparerExtensions.FromValueSelector <KeyValuePair <K, V> >(kvp => kvp.Key));
IEnumerable <KeyValuePair <K, V> > elements = Immutable
.Concat(Adaptee);
foreach (KeyValuePair <K, V> kvp in elements)
{
if (dupSet.Add(kvp))
{
yield return(kvp);
}
}
}
