let skel =
[|
[|false; false; true ; false; true |]; (* 0 is part of branch 1 *)
[|false; false; true ; false; false|]; (* 1 is the root *)
[|true ; true ; false; true ; false|]; (* 2 is the branching point *)
[|false; false; true ; false; false|]; (* 3 is branch 2 *)
[|true ; false; false; false; false|]; (* 4 is the end of branch 1 *)
|]