'Load a document from JSON.
json (JSON-data) :
A JSON-encoded document to create a new Document from.
Returns:
Document :'

@classmethod
def from_json(cls, json):

    roots_json = json['roots']
    root_ids = roots_json['root_ids']
    references_json = roots_json['references']
    references = cls._instantiate_references_json(references_json)
    cls._initialize_references_json(references_json, references)
    doc = Document()
    for r in root_ids:
        doc.add_root(references[r])
    doc.title = json['title']
    return doc
