ENSHTY ( shell, shtype, ifail )
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  Enquire shell type

Receives:
  KI_tag_shell             *shell          --- shell

Returns:
  KI_cod_ensh              *shtype         --- type-code of shell,
                                           --- from range ENSH00
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_unsuitable_entity      Not a manifold shell
  KI_general_body           general body

Description:
  The given shell is classified according to whether it bounds a solid or a
  void. Every solid body must contain exactly one solid shell, and may contain
  any number of void shells. Sheet and wire bodies must contain exactly one
  shell.

  The type of shell is returned as one of the following codes:

      ENSHSO  ---  solid
      ENSHSH  ---  sheet
      ENSHWR  ---  wire
      ENSHVO  ---  void

  This is not supported for shells of general bodies.