Vitalik's first pyethereum commit from 2013-12-19

pyethereum's Block class, introduced in a commit dated 2013-12-19 by Vitalik, stores blockchain state in a Merkle trie keyed by hexadecimal-encoded addresses. It decodes a block from RLP data, checking the balance root, contract root, transaction list hash and sibling list against their stated hashes. Methods update and read balances and contract storage, compute state and contract size, and serialize the block back into RLP. The code contains bugs, such as a sibling-root check that refers to an undefined variable and a get_state_value method that uses curnode before assignment.