split' x (u:v:ts)  =   if x `max` cost u < cost v then u:v:ts
                       else split' x (Fork u v : ts)
split' x (u:v:ts)  =   if x `max` cost u < cost v then u:v:ts
                       else split' x (Fork u v : ts)
split' x (u:v:ts)  =   if x `max` cost u < cost v then u:v:ts
                       else split' x (Fork u v : ts)