theorem Th40:
   for B be Subset of NAT holds card(B) = card((NBag1).:B)
   proof
     let B be Subset of NAT;
     reconsider F = (NBag1)|B as Function;
A1:  rng F = (NBag1).:B by RELAT_1:115;
     dom F = B by FUNCT_2:def 1;
     hence thesis by CARD_1:5,A1,WELLORD2:def 4;
   end;
