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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def document_path(cls, project, knowledge_base, document):
 """Return a fully-qualified document string."""

        return google.api_core.path_template.expand(
            'projects/{project}/knowledgeBases/{knowledge_base}/documents/{document}',
            project=project,
            knowledge_base=knowledge_base,
            document=document,
        )