TUPLE: adjacency-list
{ vertices array } ;

: <adjacency-list> ( n -- adjacency-list )
    -1 nil cons <array> adjacency-list boa ;

:: add-edge ( from to adjacency-list -- adjacency-list )
    from adjacency-list vertices>> array-nth
    dup car -1 =
    [ drop to nil cons from adjacency-list vertices>> set-array-nth ]
    [ to nil cons lappend ]
    if ;