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It means the MountPointContext is not a part of SchemaContext which holds a static information on YANG resources involved, but a 
runtime context used on data parsing. 

Schema mount support for JSON parser was requested via https://jira.opendaylight.org/browse/YANGTOOLS-1267

Analysis summary

• While schema-mounts is only supported by XML data parser for now it will require either awaiting (request) for similar functionality
  to be implemented in JSON parser or to provide own JSON parser which supports it (significant extra effort).
• CPS requires additional logic and persistence layer update in order to support YANG schema/library management requests like
  data implementing ietf-yang-schema-mount and/or ietf-yang-library
• CPS requires mechanism to store and retrieve on data parse the MountPointContext based on schema-mounts updates.

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