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 * H * A = a * (H * A)
proof
  thus a * H * A = {a} * H * A .= a * (H * A) by GROUP_2:97;
end;
