val up : ('a,'s,'g) position -> ('b,'s,'g) position option
val down : ('a,'s,'g) position -> ('b,'s,'g) position option
+val downr : ('a,'s,'g) position -> ('b,'s,'g) position option
val left : ('a,'s,'g) position -> ('b,'s,'g) position option
val right : ('a,'s,'g) position -> ('b,'s,'g) position option