user: Question: Name the written by for John Trefor
Columns: ['Series No.', 'Episode No.', 'Title', 'Directed by', 'Written by', 'UK Ratings (BBC2 Rank)', 'Original air date']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'text']
Sample row: [4.0, 1.0, '"Whitstable to the Isle of Wight "', 'David Symonds', 'n/a', '3.68m (2)', '14July2009']
SQL:

assistant: SELECT "Written by" FROM "Table" WHERE "Directed by" = 'John Trefor';