makes it possible to connect data sinks and sources.
The predecessor-member is the element in the connection that is
nearer to the source of the data. The successor-member is the element
that is further away from the source of the data. (Note that this
classification does not depend on whether the class implements
XInputStream or XOutputStream; it only
depends on the direction of data flow.)
This interface allows generic services to navigate between
arbitrary elements of a connection.
