theorem
  p=h.x & q=h.y & not y in still_not-bound_in h implies Ex(x,p) => Ex(x,
  y,q) is valid
proof
  assume p=h.x & q=h.y & not y in still_not-bound_in h;
  then All(x,p => Ex(y,q)) is valid by Th22,Th23;
  then Ex(x,p) => Ex(x,Ex(y,q)) is valid by Th35;
  hence thesis by QC_LANG2:14;
end;
