theorem Th43:
  for B for p, q, r, s st p^q = r^s & p in B & r in B
      holds p = r & q = s
proof
  let B, p, q, r, s;
  assume that A2: p^q = r^s and A3: p in B & r in B;
  consider t such that A4: p^t = r or p = r^t by Th1, A2;
  thus p = r by A3, A4, Th40;
  hence q = s by A2, FINSEQ_1:33;
end;
