MERGEN ( entity, ifail )
========================

  Remove redundant topology from entity

Receives:
  KI_tag_topology          *entity         --- entity to merge

Returns:
  KI_cod_error             *ifail          --- failure indicator

Specific errors:
  KI_none_mergeable         no mergeable entities
  KI_wrong_entity           entity of wrong type
  KI_unsuitable_entity      Mixed body
  KI_general_body           general body

Description:
  Redundant ('mergeable') edges and vertices are removed from the given
  entity, which may be a body, face, edge or vertex.

  A vertex will normally be mergeable if it is attached to only one edge, or
  if it is attached to only two edges, with the same curve geometry.

  An edge will normally be mergeable if it is a wire edge, or if the faces
  on either side have the same surface geometry.

  The allowed entity types are treated as follows:

  Vertex:
    The vertex will be removed if it is mergeable.

  Edge:
    If the edge is mergeable, it will be removed. If its vertices are then
  mergeable, they will be removed.

  Face:
    Any mergeable wire edges in the face will be removed. Then any mergeable
  vertices in or on the boundary of the face will be removed.

  Body:
    Mergeable edges and vertices are removed from the body until none remain.

    If the body contains B-curves or B-surfaces then connected sets of edges
    on each B-curve and connected sets of faces on each B-surface
    will be merged into a single edge or face provided that the B-geometry
    is capable of passing the composite geometry checks (see CHCKEN).
    However if any of the B-curves or B-surfaces are not capable
    of passing the composite geometry checks then these curves and surfaces
    will be split to produce smaller curves and surfaces that do pass the
    checks. These will then be merged as above.

  Note that the composite geometry checks will only be performed if SLIPCO
  (see SEINTP) is set to 0.

  This function is not supported for general bodies.