protected override void CloneTo (CloneContext clonectx, Expression target)
{
base.CloneTo (clonectx, target);
AQueryClause t = (AQueryClause) target;
if (block != null)
t.block = (QueryBlock) clonectx.LookupBlock (block);
if (next != null)
t.next = (AQueryClause) next.Clone (clonectx);
}