public bool Next()
{
if (_source == null)
{
_source = _sources.Dequeue();
}
if (_source.Next())
{
return(true);
}
_consumed.Enqueue(_source);
if (_sources.Count <= 0)
{
return(false);
}
_source = _sources.Dequeue();
if (_source == null)
{
return(false);
}
return(_source.Next());
}