theorem Th21:
  <%z0,z1,z2%>.0 = z0
  proof
A1: dom 0_.L = NAT by FUNCOP_1:13;
    thus <%z0,z1,z2%>.0 = (0_.L+*(0,z0)+*(1,z1)).0 by FUNCT_7:32
    .= (0_.L+*(0,z0)).0 by FUNCT_7:32
    .= z0 by A1,FUNCT_7:31;
  end;
