user: Question: How many different pairs of writers wrote the episode with series number 56?
Columns: ['No. in series', 'No. in season', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code']
Types: ['real', 'real', 'text', 'text', 'text', 'text', 'real']
Sample row: [47.0, 1.0, '"Knight of the Drones (Part 1)"', 'Sidney Hayers', 'Robert Foster & Gerald Sanford', 'September30,1984', 58675.0]
SQL:

assistant: SELECT COUNT("Written by") FROM "Table" WHERE "No. in series" = 56.0;