theorem Th23:
  <%z0,z1,z2%>.2 = z2
  proof
    dom(0_.L+*(0,z0)+*(1,z1)) = dom(0_.L+*(0,z0)) by FUNCT_7:30
    .= dom 0_.L by FUNCT_7:30
    .= NAT by FUNCOP_1:13;
    hence <%z0,z1,z2%>.2 = z2 by FUNCT_7:31;
  end;
