Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_31598 ( "Schemas" text, "Flexible" text, "Semantic verification" text, "Dictionary" text, "Information Model" text, "Synonyms and homonyms" text, "Dialecting" text, "Web standard" text, "Transformations" text, "Lightweight" text, "Human readable" text )

### Question ###
What is the lightweight value with no information model and the flexible value is unknown?

### Accurate SQL ###

SELECT "Lightweight" FROM table_31598 WHERE "Information Model" = 'no' AND "Flexible" = 'unknown'