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 Th9:
  A * a * H = A * (a * H)
proof
  thus A * a * H = A * ({a} * H) by GROUP_2:96
    .= A * (a * H);
end;
