theorem Th7:
  x in A & x <> <%>E & n > 0 implies A |^ n <> {<%>E}
proof
  assume that
A1: x in A & x <> <%>E and
A2: n > 0;
  A <> {<%>E} by A1,TARSKI:def 1;
  hence thesis by A2,FLANG_1:29;
end;
