let x be object ; :: according to GLIB_014:def 1 :: thesis: ( x in X \/ Y implies x is _Graph )
assume x in X \/ Y ; :: thesis: x is _Graph

per cases then ( x in X or x in Y ) by XBOOLE_0:def 3;

suppose x in X ; :: thesis: x is _Graph

hence x is _Graph by Def1; :: thesis: verum

end;

suppose x in Y ; :: thesis: x is _Graph

hence x is _Graph by Def1; :: thesis: verum

end;

end;