http://invest5gh6ln6dom3z2rgtkbctdmpmcotyggb376jtbrfbqp6i2u36yd.onion/bitcoin-python/index15.html
One then makes a contract with the rule that, every N blocks, the contract would pick a random index in the Merkle tree (using the previous block hash, accessible from contract code, as a source of randomness), and give X ether to the first entity to supply a transaction with a simplified payment verification-like proof of ownership of the block at that particular index in the tree.