let x be Element of X; :: thesis: x is DWalk of G

x in { W where W is DWalk of G : verum } ;

then ex y being DWalk of G st y = x ;

hence x is DWalk of G ; :: thesis: verum

x in { W where W is DWalk of G : verum } ;

then ex y being DWalk of G st y = x ;

hence x is DWalk of G ; :: thesis: verum