theorem Th38:
  W1 c= W2 iff [#]W1 c= [#]W2
proof
  thus W1 c= W2 implies [#]W1 c= [#]W2 by Th36;
  assume [#]W1 c= [#]W2;
  then for a st a in W1 holds a in W2;
  hence thesis by Th37;
end;
