code: def concatenate(boxlists, fields=None):
      """Concatenate list of BoxLists.
  
      This op concatenates a list of input BoxLists into a larger BoxList.  It also
      handles concatenation of BoxList fields as long as the field tensor shapes
      are equal except for the first dimension.
  
      Args:
        boxlists: list of BoxList objects
        fields: optional list of fields to also concatenate.  By default, all
          fields from the first BoxList in the list are included in the
          concatenation.
  
      Returns:
        a BoxList with number of boxes equal to
          sum([boxlist.num_boxes() for boxlist in BoxList])
      Raises:
        ValueError: if boxlists is invalid (i.e., is not a list, is empty, or
          contains non BoxList objects), or if requested fields are not contained in
          all boxlists
      """
      if not isinstance(boxlists, list):
          raise ValueError("boxlists should be a list")
      if not boxlists:
          raise ValueError("boxlists should have nonzero length")
      for boxlist in boxlists:
          if not isinstance(boxlist, np_box_list.BoxList):
              raise ValueError(
                  "all elements of boxlists should be BoxList objects"
              )
      concatenated = np_box_list.BoxList(
          np.vstack([boxlist.get() for boxlist in boxlists])
      )
      if fields is None:
          fields = boxlists[0].get_extra_fields()
      for field in fields:
          first_field_shape = boxlists[0].get_field(field).shape
          first_field_shape = first_field_shape[1:]
          for boxlist in boxlists:
              if not boxlist.has_field(field):
                  raise ValueError(
                      "boxlist must contain all requested fields"
                  )
              field_shape = boxlist.get_field(field).shape
              field_shape = field_shape[1:]
              if field_shape != first_field_shape:
                  raise ValueError(
                      "field %s must have same shape for all boxlists "
                      "except for the 0th dimension." % field
                  )
          concatenated_field = np.concatenate(
              [boxlist.get_field(field) for boxlist in boxlists], axis=0
          )
          concatenated.add_field(field, concatenated_field)
      return concatenated
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  import numpy
  from .None import np_box_list, np_box_ops
label_desc: Use image node_id 13 for calling a global function with example usage: concatenate(boxlists, fields) and returns: concatenated