code: public NodeList validate(Document doc, Document instruction)
  			throws Exception {
  		return schemaValidation(doc, instruction).toNodeList();
  	}
docs: Validates the given document against the schema references supplied in
  the accompanying instruction document.
  
  @param doc
  The document to be validated.
  @param instruction
  A document containing schema references; may be null, in which
  case embedded schema references will be used instead.
  @return A list of Element nodes ({@code <error>}) containing error
  messages.
  @throws Exception
  If any error occurs.