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

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

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

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

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

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

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