def _describe_style(which_styles):
  """Returns a string describing a linear combination of styles."""
  def _format(v):
    formatted = str(int(round(v * 1000.0)))
    while len(formatted) < 3:
      formatted = '0' + formatted
    return formatted

  values = []
  for k in sorted(which_styles.keys()):
    values.append('%s_%s' % (k, _format(which_styles[k])))
  return '_'.join(values)