Posting Puzzle

Summary

Push one of three trays to add to both rows at once, matching envelope and stamp pairs into the mailbox while mismatches return. Twelve stages are instances of Post's Correspondence Problem: the first six are solvable by hand, the last six are hard cases taken from research.

• Twelve stages based on Post's Correspondence Problem
• One rule: push a tray, both rows grow
• Undo, reset and stage-select keys built in