reserve x, r for Real;
reserve A for symmetrical Subset of COMPLEX;
reserve F,G for PartFunc of REAL, REAL;

theorem
  F is even & G is even & dom F /\ dom G is symmetrical implies F + G is even
proof
  assume that
A1: F is even and
A2: G is even and
A3: dom F /\ dom G is symmetrical;
A4: dom F /\ dom G=dom (F + G) by VALUED_1:def 1;
  then
A5: dom (F + G) c= dom G by XBOOLE_1:17;
A6: dom (F + G) c= dom F by A4,XBOOLE_1:17;
  for x st x in dom(F + G) & -x in dom(F + G) holds (F + G).(-x)=(F + G).x
  proof
    let x;
    assume that
A7: x in dom(F + G) and
A8: -x in dom(F + G);
    (F + G).(-x)=F.(-x) + G.(-x) by A8,VALUED_1:def 1
      .=F.x + G.(-x) by A1,A6,A7,A8,Def3
      .=F.x + G.x by A2,A5,A7,A8,Def3
      .=(F + G).x by A7,VALUED_1:def 1;
    hence thesis;
  end;
  then (F + G) is with_symmetrical_domain quasi_even by A3,A4;
  hence thesis;
end;
