theorem
  Ex(x,Ex(y,p)) => Ex(y,Ex(x,p)) is valid & Ex(x,y,p) => Ex(y,x,p) is valid
proof
  not x in still_not-bound_in Ex(x,p) by Th6;
  then
A1: not x in still_not-bound_in Ex(y,Ex(x,p)) by Th6;
  All(y,p => Ex(x,p)) is valid & All(y,p => Ex(x,p)) => (Ex(y,p) => Ex(y,
  Ex(x, p))) is valid by Th15,Th23,Th34;
  then Ex(y,p) => Ex(y,Ex(x,p)) is valid by CQC_THE1:65;
  hence Ex(x,Ex(y,p)) => Ex(y,Ex(x,p)) is valid by A1,Th19;
  then Ex(x,y,p) => Ex(y,Ex(x,p)) is valid by QC_LANG2:14;
  hence thesis by QC_LANG2:14;
end;
