reserve x,X,Y for set;
reserve C for non empty set;
reserve c for Element of C;
reserve V for RealNormSpace;
reserve f,f1,f2,f3 for PartFunc of C,V;
reserve r,r1,r2,p for Real;

theorem
  f1|X is constant & f2|Y is constant implies (f1+f2)|(X /\ Y) is
  constant & (f1-f2)|(X /\ Y) is constant
proof
  assume that
A1: f1|X is constant and
A2: f2|Y is constant;
  consider r1 being VECTOR of V such that
A3: for c st c in X /\ dom f1 holds (f1/.c) = r1 by A1,PARTFUN2:35;
  consider r2 being VECTOR of V such that
A4: for c st c in Y /\ dom f2 holds (f2/.c) = r2 by A2,PARTFUN2:35;
  now
    let c;
    assume
A5: c in X /\ Y /\ dom (f1+f2);
    then
A6: c in X /\ Y by XBOOLE_0:def 4;
    then
A7: c in X by XBOOLE_0:def 4;
A8: c in dom (f1+f2) by A5,XBOOLE_0:def 4;
    then
A9: c in (dom f1 /\ dom f2) by Def1;
    then c in dom f1 by XBOOLE_0:def 4;
    then
A10: c in X /\ dom f1 by A7,XBOOLE_0:def 4;
A11: c in Y by A6,XBOOLE_0:def 4;
    c in dom f2 by A9,XBOOLE_0:def 4;
    then
A12: c in Y /\ dom f2 by A11,XBOOLE_0:def 4;
    thus (f1+f2)/.c = (f1/.c) + (f2/.c) by A8,Def1
      .= r1 + (f2/.c) by A3,A10
      .= r1 + r2 by A4,A12;
  end;
  hence (f1+f2)|(X /\ Y) is constant by PARTFUN2:35;
  now
    let c;
    assume
A13: c in X /\ Y /\ dom (f1-f2);
    then
A14: c in X /\ Y by XBOOLE_0:def 4;
    then
A15: c in X by XBOOLE_0:def 4;
A16: c in dom (f1-f2) by A13,XBOOLE_0:def 4;
    then
A17: c in (dom f1 /\ dom f2) by Def2;
    then c in dom f1 by XBOOLE_0:def 4;
    then
A18: c in X /\ dom f1 by A15,XBOOLE_0:def 4;
A19: c in Y by A14,XBOOLE_0:def 4;
    c in dom f2 by A17,XBOOLE_0:def 4;
    then
A20: c in Y /\ dom f2 by A19,XBOOLE_0:def 4;
    thus (f1-f2)/.c = (f1/.c) - (f2/.c) by A16,Def2
      .= r1 - (f2/.c) by A3,A18
      .= r1 - r2 by A4,A20;
  end;
  hence thesis by PARTFUN2:35;
end;
