theorem
  ex t st x.t = x
proof
  consider i,t being object such that
A1: i in {4} and
A2: t in QC-symbols(A) and
A3: [i,t] = x by ZFMISC_1:def 2;
  reconsider t as QC-symbol of A by A2;
  take t;
  thus thesis by A1,A3,TARSKI:def 1;
end;
