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

proof

let x, y be set ; :: thesis: ( [x,y] in (X \/ Y) | R iff [x,y] in (X | R) \/ (Y | R) )
A1: ( y in X \/ Y iff ( y in X or y in Y ) ) by XBOOLE_0:def 3;
A2: ( [x,y] in (X | R) \/ (Y | R) iff ( [x,y] in X | R or [x,y] in Y | R ) ) by XBOOLE_0:def 3;
( [x,y] in (X \/ Y) | R iff ( y in X \/ Y & [x,y] in R ) ) by Def12;
hence ( [x,y] in (X \/ Y) | R iff [x,y] in (X | R) \/ (Y | R) ) by A1, A2, Def12; :: thesis: verum

end;

hence (X \/ Y) | R = (X | R) \/ (Y | R) by Def2; :: thesis: verum