Related Syntax
operator	is referent

Return type: Boolean
Returns:       

Returns true if the stream is attached to a referent and false otherwise. Replacing is with isnt will reverse the results.

  stream-name  indexer? (is | isnt) referent

Tests whether the specified stream is attached to a referent.

Note that if a stream is bound to a previously defined referent and then discarded, that referent still exists, but is no longer attached.