reserve T for non empty Abelian
  add-associative right_zeroed right_complementable RLSStruct,
  X,Y,Z,B,C,B1,B2 for Subset of T,
  x,y,p for Point of T;

theorem
  X (-) (Y (+) Z) = (X (-) Z) (-) Y
proof
  X (-) (Y (+) Z) = X (-) (Z (+) Y) by Th12
    .=X (-) Z (-) Y by Th23;
  hence thesis;
end;
