code: def add_annotation_value(graph, annotation, value):
      
      if annotation not in graph.defined_annotation_keywords:
          raise ValueError('annotation not defined: {}'.format(annotation))
  
      for u, v, k in graph.edges(keys=True):
          if ANNOTATIONS not in graph[u][v][k]:
              continue
  
          if annotation not in graph[u][v][k][ANNOTATIONS]:
              graph[u][v][k][ANNOTATIONS][annotation] = {}
  
          graph[u][v][k][ANNOTATIONS][annotation][value] = True
docs: Add the given annotation/value pair to all qualified edges.
  
      :param pybel.BELGraph graph:
      :param str annotation:
      :param str value: