theorem Th9:
  for k being Nat, P being QC-pred_symbol of k,A, ll being
  QC-variable_list of k,A holds Sub_P(P,ll,e) = [P!ll,e]
proof
  let k be Nat, P be QC-pred_symbol of k,A,
ll be QC-variable_list of k, A;
  set QCP = {QP where QP is QC-pred_symbol of A: the_arity_of QP = k };
  P in QCP;
  then
A1: ex Q being QC-pred_symbol of A st P = Q & the_arity_of Q = k;
  len ll = k by CARD_1:def 7;
  hence thesis by A1,Def18;
end;
