Two clues

A spam filter looks for clues: the word “free”, a link to a lottery, a sender you’ve never written to. Before any clue, an email is spam with chance base_chance. Each clue alone, starting from that same base, would make it spam with chance clue_chances[i]. The clues are independent: each one is separate evidence. Write combine(base_chance, clue_chances): the chance it’s spam given all the clues.