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

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

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

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

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

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

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