PK_ERROR_code_t        PK_SHELL_find_sign
(
--- received arguments ---
PK_SHELL_t             shell,       --- a shell

--- returned arguments ---
PK_SHELL_sign_t *const shell_sign   --- the sign of the shell
)


This function classifies the given shell according to whether it
closes off a volume and, if it does, whether the volume inside the
shell is finite or infinite.


The shell classification will be one of:

    PK_SHELL_sign_positive_c  There is a subset of the faces of the shell
                              which divides space into two volumes, the
                              volume inside the shell being finite; for
                              example, the outermost shell of a solid region.

    PK_SHELL_sign_negative_c  There is a subset of the faces of the shell
                              which divides space into two volumes, the
                              volume inside the shell being infinite; for
                              example, an inner shell of a solid region.

    PK_SHELL_sign_open_c      There is no subset of faces of the shell
                              which divides space into two volumes, in
                              other words, all points are either in or on
                              the shell; for example, any shell consisting
                              only of wireframe edges.