let X, Y be set ; :: thesis: for R being Relation holds R | (X \/ Y) = (R | X) \/ (R | Y)
let R be Relation; :: thesis: R | (X \/ Y) = (R | X) \/ (R | Y)
let x be set ; :: according to RELAT_1:def 2 :: thesis: for b being set holds
( [x,b] in R | (X \/ Y) iff [x,b] in (R | X) \/ (R | Y) )
let y be set ; :: thesis: ( [x,y] in R | (X \/ Y) iff [x,y] in (R | X) \/ (R | Y) )

hereby :: thesis: ( [x,y] in (R | X) \/ (R | Y) implies [x,y] in R | (X \/ Y) )

assume A8: [x,y] in R | (X \/ Y) ; :: thesis: [x,y] in (R | X) \/ (R | Y)
then x in X \/ Y by Def11;
then A9: ( x in X or x in Y ) by XBOOLE_0:def 3;
[x,y] in R by A8, Def11;
then ( [x,y] in R | X or [x,y] in R | Y ) by A9, Def11;
hence [x,y] in (R | X) \/ (R | Y) by XBOOLE_0:def 3; :: thesis: verum

end;

assume A1: [x,y] in (R | X) \/ (R | Y) ; :: thesis: [x,y] in R | (X \/ Y)

per cases ( [x,y] in R | Y or [x,y] in R | X ) by A1, XBOOLE_0:def 3;

suppose A3: [x,y] in R | Y ; :: thesis: [x,y] in R | (X \/ Y)

then x in Y by Def11;
then A4: x in X \/ Y by XBOOLE_0:def 3;
[x,y] in R by A3, Def11;
hence [x,y] in R | (X \/ Y) by A4, Def11; :: thesis: verum

end;

suppose A6: [x,y] in R | X ; :: thesis: [x,y] in R | (X \/ Y)

then x in X by Def11;
then A7: x in X \/ Y by XBOOLE_0:def 3;
[x,y] in R by A6, Def11;
hence [x,y] in R | (X \/ Y) by A7, Def11; :: thesis: verum

end;

end;