class_docstr: Defined by J. Welles Wilder, Jr. in 1978 in his book *"New Concepts in
  Technical Trading Systems"* as part of the Directional Move System to
  calculate Directional Indicators.
  
  Positive if the given data has moved lower than the previous day
  
  Formula:
    - downmove = data(-1) - data
  
  See:
    - https://en.wikipedia.org/wiki/Average_directional_movement_index
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code: def __init__(self):
      self.lines.downmove = self.data(-1) - self.data
      super(DownMove, self).__init__()
  
imports: from __future__ import absolute_import, division, print_function, unicode_literals
  from .None import Indicator, And, If, MovAv, ATR
label_desc: Use image node_id 1 to create a new DownMove object from inherited base classes: Indicator with example: obj = DownMove()