Add an element to the <keyword>'s list of deviations.

  Can be used by plugins that add support for specific extension
  statements.

def add_deviation_element(keyword, element):
  """Add an element to the <keyword>'s list of deviations.

  Can be used by plugins that add support for specific extension
  statements."""
  if keyword in _valid_deviations:
      _valid_deviations[keyword].append(element)
  else:
      _valid_deviations[keyword] = [element]