Unlike attr<-, this method (essentially) stores the
attribute in a reference to the object, not a copy. As such it can
be used within function call (handlers) to assign values outside
the scope of the function call.
tag(obj, key)# S3 method for default
tag(obj, key)
tag(obj, key) <- value
tag(obj, key) <- value
object
character. Values are stored by key. If missing, all keys are returned.
to assign to key