theorem Th28:
  N is add-associative right_zeroed right_complementable implies
  <% a1,b1 %> - <% a2,b2 %> = <% a1-a2,b1-b2 %>
  proof
    assume N is add-associative right_zeroed right_complementable;
    hence <%a1,b1%> - <%a2,b2%> = <%a1,b1%>+<%-a2,-b2%> by Th27
    .= <%a1-a2,b1-b2%> by Def9;
  end;
