This Stream subclass is only used
inside a Spanner object to provide object storage
of connected elements (things the Spanner spans).

This subclass name can be used to search in an
object's .sites and find any and all
locations that are SpannerStorage objects.

A `client` keyword argument must be provided by the Spanner in creation.

>>> stream.SpannerStorage(client=spanner.Slur())
<music21.stream.SpannerStorage for music21.spanner.Slur>

* Changed in v8: spannerParent is renamed client.
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