reserve x,y for set,
  G for Group,
  A,B,H,H1,H2 for Subgroup of G,
  a,b,c for Element of G,
  F,F1 for FinSequence of the carrier of G,
  I,I1 for FinSequence of INT,
  i,j for Element of NAT;

theorem Th4:
  ([.a,b".]|^ b)" = [.b",a.]|^ b
proof
  thus ([.a,b".]|^ b)" = [.a,b".]"|^ b by GROUP_3:26
                 .= [.b",a.]|^ b by GROUP_5:22;
end;
