reserve x,y,y1,y2 for set;
reserve G for Group;
reserve a,b,c,d,g,h for Element of G;
reserve A,B,C,D for Subset of G;
reserve H,H1,H2,H3 for Subgroup of G;
reserve n for Nat;
reserve i for Integer;

theorem
  a in con_class b implies b in con_class a
proof
  assume a in con_class b;
  then a,b are_conjugated by Th81;
  hence thesis by Th81;
end;
