reserve M for non empty set;
reserve V for ComplexNormSpace;
reserve f,f1,f2,f3 for PartFunc of M,V;
reserve z,z1,z2 for Complex;

theorem
  f1 + (f2 - f3) = f1 + f2 - f3
proof
A1: dom (f1 + (f2 - f3)) = dom f1 /\ dom (f2 - f3) by VFUNCT_1:def 1
    .= dom f1 /\ (dom f2 /\ dom f3) by VFUNCT_1:def 2
    .= dom f1 /\ dom f2 /\ dom f3 by XBOOLE_1:16
    .= dom (f1 + f2) /\ dom f3 by VFUNCT_1:def 1
    .= dom (f1 + f2 - f3) by VFUNCT_1:def 2;
  now
    let c be Element of M;
    assume
A2: c in dom (f1 + (f2 - f3));
    then c in dom f1 /\ dom (f2 - f3) by VFUNCT_1:def 1;
    then
A3: c in dom (f2 - f3) by XBOOLE_0:def 4;
    c in dom (f1 + f2) /\ dom f3 by A1,A2,VFUNCT_1:def 2;
    then
A4: c in dom (f1 + f2) by XBOOLE_0:def 4;
    thus (f1 + (f2 - f3))/.c = (f1/.c) + ((f2 - f3)/.c) by A2,VFUNCT_1:def 1
      .= (f1/.c) + ((f2/.c) - (f3/.c)) by A3,VFUNCT_1:def 2
      .= (f1/.c) + (f2/.c) - (f3/.c) by RLVECT_1:def 3
      .= ((f1 + f2)/.c) - (f3/.c) by A4,VFUNCT_1:def 1
      .= (f1 + f2 - f3)/.c by A1,A2,VFUNCT_1:def 2;
  end;
  hence thesis by A1,PARTFUN2:1;
end;
