Build Stack Overflow

• Users post questions (title, body, tags), answers and comments on either; each has an author and a timestamp
• Users can upvote or downvote a question or answer once; changing or retracting a vote is allowed, authors cannot vote on their own posts, and a score is maintained
• Reputation changes with events (for example +10 for an upvoted answer, +5 for an upvoted question, -2 for a downvote received, +15 when an answer is accepted), and downvoting requires a minimum reputation
• The question author can accept exactly one answer; accepting another replaces the first and adjusts reputation both ways
• search({ text?, tags?, sort }) finds questions by words in the title or body and by tags (all tags must match), and sorts by newest, most votes or unanswered
• Tags are normalised (lowercase, no duplicates), and every operation validates that the referenced entities exist

Only after every requirement works.

• Edit history and moderation (close, delete)
• Bounty on a question
• Tag following and notifications through an observer