PIERCE ( face, ifail )
======================

  Remove face from sheet

Receives:
  KI_tag_face              *face           --- face to remove

Returns:
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_bad_wire               invalid wire would result
  KI_fragment               sheet would break apart
  KI_not_sheet              body is not a sheet

Description:

  This routine deletes a face from a sheet body. The given face must be
  a face of a sheet body, with or without a surface attached. Removal of
  the face must not break the sheet into two or more parts.

  If the body has more than one face, it remains a sheet body; edges and
  vertices belonging to the face, but which are shared with no other
  face, will be deleted along with it.

  If the face is the only face of the sheet body, the result will be
  either a wire or minimal body; none of the body's edges and vertices
  will be deleted by the operation. The edges and vertices must form a
  valid wire or minimal body, i.e. they must be connected (in a single
  loop or a pair of bi-wire loops) and no vertex may be used by more
  than two edges.