reserve i,k for Nat;
reserve A for QC-alphabet;
reserve x for bound_QC-variable of A;
reserve a for free_QC-variable of A;
reserve p,q for Element of QC-WFF(A);
reserve l for FinSequence of QC-variables(A);
reserve P,Q for QC-pred_symbol of A;
reserve V for non empty Subset of QC-variables(A);
reserve s,t for QC-symbol of A;

theorem
  ex i st (A)a.i = a
proof
  consider x,y being object such that
A1: x in {6} and
A2: y in NAT and
A3: [x,y] = a by ZFMISC_1:def 2;
  reconsider i = y as Nat by A2;
  take i;
  thus thesis by A1,A3,TARSKI:def 1;
end;
