code: def is_subfeature_of (parent_property, f):
      
      if __debug__:
          from .property import Property
          assert isinstance(parent_property, Property)
          assert isinstance(f, Feature)
  
      if not f.subfeature:
          return False
  
      p = f.parent
      if not p:
          return False
  
      parent_feature = p[0]
      parent_value = p[1]
  
      if parent_feature != parent_property.feature:
          return False
  
      if parent_value and parent_value != parent_property.value:
          return False
  
      return True
docs: Return true iff f is an ordinary subfeature of the parent_property's
          feature, or if f is a subfeature of the parent_property's feature
          specific to the parent_property's value.