private static Func<dynamic> GetDerivedUnknown(this Func<dynamic> fact, Func<dynamic> unknown)
{
if (unknown.LeftMatchesLeft(fact))
return unknown.CalculateDerivedFrom(fact);
if (unknown.LeftMatchesLeft(fact.Invert()))
return unknown.CalculateDerivedFrom(fact.Invert());
return null;
}