reserve o,m for set;
reserve C for Cartesian_category;
reserve a,b,c,d,e,s for Object of C;
reserve C for Cocartesian_category;
reserve a,b,c,d,e,s for Object of C;

theorem Th60:
  dom in2(a,b) = b & cod in2(a,b) = a+b
proof
  set i1 = (the Incl1 of C).(a,b), i2 = (the Incl2 of C).(a,b);
  a+b is_a_coproduct_wrt i1,i2 by Def26;
  hence thesis by Def26;
end;
