Fix operation 'Document.filter' when using 'condition' parameter with a list of document as input

When expression from parameter 'condition' is evaluated, the expression's context has no knowledge of the document being iterated, making it impossible to filter a list of documents.
Provided Document['dc:format'] == 'text' is a valid (?) expression for the condition parameter, when operation loops on list of documents, no variable Document (or other) in the expression's context is properly defined to allow a proper evaluation on each document of the list.