user: Question: Name the reason for 20 July 1031
Columns: ['Co-king', 'Relationship to Monarch', 'Crowned', 'Co-kingship ceased', 'Reason', 'Monarch']
Types: ['text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Robert (II)', 'son', '30 December 987', '24 October 996', 'became sole king', 'Hugh Capet']
SQL:

assistant: SELECT "Reason" FROM "Table" WHERE "Co-kingship ceased" = '20 July 1031';