2011-05-14 ago slightly more efficient claset operations, using Item_Net to maintain rules in canonical order;
2010-04-23 ago Item_Net/Named_Thms: export efficient member operation;
2009-11-16 ago member/cons: slightly more correct treatment of multi-index, notably empty one;
2009-11-01 ago allow multi-index;
2009-03-17 ago turned structure NetRules into general Item_Net, which is loaded earlier;