code: def add_parser(parser):
      """
      Add a chord evaluation sub-parser to an existing parser.
  
      Parameters
      ----------
      parser : argparse parser instance
          Existing argparse parser object.
  
      Returns
      -------
      sub_parser : argparse sub-parser instance
          Chord evaluation sub-parser.
  
      """
      import argparse
  
      # add chord evaluation sub-parser to the existing parser
      p = parser.add_parser(
          "chords",
          help="chord evaluation",
          formatter_class=argparse.RawDescriptionHelpFormatter,
          description="""
      This program evaluates pairs of files containing the chord annotations and
      predictions. Suffixes can be given to filter them from the list of files.
  
      Each line represents a chord and must have the following format with values
      being separated by whitespace (chord_label follows the syntax as defined
      by Harte 2010):
      `start_time end_time chord_label`
      """,
      )
      # set defaults
      p.set_defaults(
          eval=ChordEvaluation,
          sum_eval=ChordSumEvaluation,
          mean_eval=ChordMeanEvaluation,
          load_fn=load_chords,
      )
      # file I/O
      evaluation_io(p, ann_suffix=".chords", det_suffix=".chords.txt")
      # return the sub-parser and evaluation argument group
      return p
  
imports: import numpy
  from .None import evaluation_io, EvaluationMixin
  from ..io import load_chords
label_desc: Use image node_id 19 for calling a global function with example usage: add_parser(parser) and returns: p