theorem Th28:
  f is_a_proof_wrt X implies Effect(f) in Cn(X)
proof
  assume
A1: f is_a_proof_wrt X;
then A2: 1 <= len f by Th21;
then A3: (f.(len f))`1 in Cn(X) by A1,Th27;
 f <> {} by A2;
  hence thesis by A3,Def6;
end;
