user: Question: Name the mixed doubles for year before 1991 and men's doubles of David Jepson Buck Ryan
Columns: ['Year', "Men's singles", "Women's singles", "Men's doubles", "Women's doubles", 'Mixed doubles']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1971.0, 'Kenneth Wain', 'Barbara Martin', 'Alfred Cefai Kenneth Wain', 'Carrol McKenzie Hazel Tough', 'John Farmer Barbara Martin']
SQL:

assistant: SELECT "Mixed doubles" FROM "Table" WHERE "Year" < 1991.0 AND "Men's doubles" = 'David Jepson Buck Ryan';