SUBBYS ( targby, tolbys, assemb, nbodys, ifail )
================================================

  Subtract bodies

Receives:
  KI_tag_body              *targby         --- body to be modified
  KI_tag_list_body         *tolbys         --- body or list of bodies
                                           --- used to modify 'targby'

Returns:
  KI_tag_assembly          *assemb         --- assembly of resulting bodies
 <KI_int_nitems>           *nbodys         --- number of bodies in 'assemb'
  KI_cod_error             *ifail          --- failure code

Specific errors:
  KI_invalid_bodies         Boolean failure or invalid bodies
  KI_partial_coi_found      Boolean failure due to partial coincidence
  KI_wire_body              Target or tool is a wire body
  KI_missing_geom           Target or tool has incomplete geometry
  KI_non_manifold           Non-manifold result
  KI_same_tool_and_target   Tool body is also target body
  KI_mixed_sheets_solids    Mixture of sheet and solid tool bodies
  KI_instanced_tools        Instanced tool bodies
  KI_duplicate_tools        Duplication in list of tool bodies
  KI_not_in_same_partition  Target and tools are not all in the same partition
  KI_unsuitable_entity      mixed body
  KI_general_body           general body

Description:
  The target body (workpiece) is modified by removal of all
  regions where it overlaps one of the tool bodies (modifiers).
  The resulting body or bodies replace the workpiece in the
  world and are instanced in the new assembly returned.

  The modifiers are deleted and the tags in the list 'tolbys'
  become dead.

  If 'targby' was instanced in any assemblies, the instances
  are now of 'assemb'.

  Boolean operations such as subtraction cannot be performed
  unless the bodies are completely specified geometrically.

  This function does not support general bodies.