theorem Th19:
  F is atomic implies (@F.1)`1 <> 0 & (@F.1)`1 <> 1 & (@F.1)`1 <>
  2 & (@F.1)`1 <> 3
proof
  assume F is atomic;
  then ex k being Nat st @F.1 is QC-pred_symbol of k, A by Th18;
  hence thesis by Th17;
end;
