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 Th16:
  N ` A c= A
proof
  let x be object;
  assume x in N ` A;
  then consider y being Element of G such that
A1: y = x & y * N c= A;
  y in y * N by GROUP_2:108;
  hence thesis by A1;
end;
