code: func (as *AttributeSource) Add(s string) Attribute {
  	attImpl, ok := as.attributes[s]
  	if !ok {
  		attImpl = as.factory.Create(s)
  		as.AddImpl(attImpl)
  	}
  	return attImpl
  }
docs: /*
  The caller must pass in a Attribute instance. This method first
  checks if an instance of that type is already in this AttributeSource
  and returns it. Otherwise a new instance is created, added to this
  AttributeSource and returned.
  */