theorem Th71:
  for p,q being Function , A being set holds (p +* q)|A = p|A +* q|A
proof
  let p,q be Function , A be set;
A1: dom ((p +* q)|A) = dom (p +* q) /\ A by RELAT_1:61
    .= (dom p \/ dom q) /\ A by Def1
    .= (dom p /\ A) \/ (dom q /\ A) by XBOOLE_1:23
    .= dom (p|A) \/ (dom q /\ A) by RELAT_1:61
    .= dom (p|A) \/ dom (q|A) by RELAT_1:61;
  for x being object st x in dom (p|A) \/ dom (q|A) holds (x in dom (q|A)
implies ((p +* q)|A).x = (q|A).x) & (not x in dom (q|A) implies ((p +* q)|A).x
  = (p|A).x)
  proof
    let x be object;
    assume
A2: x in dom (p|A) \/ dom (q|A);
    then x in dom (p|A) or x in dom (q|A) by XBOOLE_0:def 3;
    then x in (dom p /\ A) or x in dom q /\ A by RELAT_1:61;
    then
A3: x in A by XBOOLE_0:def 4;
    hereby
      assume
A4:   x in dom (q|A);
      then x in (dom q /\ A) by RELAT_1:61;
      then
A5:   x in dom q by XBOOLE_0:def 4;
      thus ((p +* q)|A).x = (p +* q).x by A1,A2,FUNCT_1:47
        .= q.x by A5,Th13
        .= (q|A).x by A4,FUNCT_1:47;
    end;
    assume
A6: not x in dom (q|A);
    then not x in (dom q /\ A) by RELAT_1:61;
    then
A7: not x in dom q by A3,XBOOLE_0:def 4;
A8: x in dom (p|A) by A2,A6,XBOOLE_0:def 3;
    then x in dom p /\ A by RELAT_1:61;
    then x in dom p by XBOOLE_0:def 4;
    then x in dom (p +* q) by Th12;
    then x in dom (p +* q) /\ A by A3,XBOOLE_0:def 4;
    then x in dom ((p +* q)|A) by RELAT_1:61;
    hence ((p +* q)|A).x = (p +* q).x by FUNCT_1:47
      .= p.x by A7,Th11
      .= (p|A).x by A8,FUNCT_1:47;
  end;
  hence thesis by A1,Def1;
end;
