code: def is_implicit_value (value_string):
      
      assert isinstance(value_string, basestring)
      if value_string in __implicit_features:
          return __implicit_features[value_string]
  
      v = value_string.split('-')
  
      if v[0] not in __implicit_features:
          return False
  
      feature = __implicit_features[v[0]]
  
      for subvalue in (v[1:]):
          if not __find_implied_subfeature(feature, subvalue, v[0]):
              return False
  
      return True
docs: Returns true iff 'value_string' is a value_string
      of an implicit feature.