theorem Th17:
  for X being set st X c= u holds X is Element of SubstLatt (V, C)
proof
  let X be set;
  reconsider u9 = u as Element of SubstitutionSet (V, C) by SUBSTLAT:def 4;
  assume X c= u;
  then X c= u9;
  then X in SubstitutionSet (V, C) by Lm4;
  hence thesis by SUBSTLAT:def 4;
end;
