// modify this method as shown
public MyImmutable Apply(Func <decimal, decimal, decimal> aggFunc, MyImmutable y)
{
var aggregated = new Dictionary <ImmutableKey, decimal>(AllKeys.Count());
foreach (ImmutableKey bt in AllKeys)
{
aggregated[bt] = aggFunc(this[bt], y[bt]);
}
return(GetNew(aggregated));
}
