reserve G for Group;
reserve A,B for non empty Subset of G;
reserve N,H,H1,H2 for Subgroup of G;
reserve x,a,b for Element of G;

theorem Th30:
  for x being Element of G st x in N ~ (N ` (N ~ A))
  holds x * N meets N ` (N ~ A)
proof
  let x be Element of G;
  assume x in N ~ (N ` (N ~ A));
  then ex x1 being Element of G st x = x1 & x1 * N meets N ` (N ~ A);
  hence thesis;
end;
