chain object returns one of the possible topological
orders, through a permutation when
is.null(ord). If not
ord must be a proposed order to be checked given as a
is.numeric(ord) or a vector of ordered names if
Indicates what must be done.
For the moment the
ord option is bad and an error message is
returned when used.
a permutation vector of the nodes of the /nbn/ or a named character with the nodes not having their parents before them; when it is of length zero this means that the check was successful.