From the paper [1]: > Essentially, an SPV proof is composed of (a) a list of blockheaders demonstrating proof-of-work, and (b) a cryptographic proof that an ou

From the paper [1]: > Essentially, an SPV proof is composed of (a) a list of blockheaders demonstrating proof-of-work, and (b) a cryptographic proof that an output was created in one of the blocks in the list. This allows verifiers to check that some amount of work has been committed to the existence of an output. Such a proof may be invalidated by another proof demonstrating the existence of a chain with more work which does not include the block which created the output. which reads to me like embedding a full copy of the rules engine of one blockchain inside the other, meaning 1) all chains likely need to be Turing complete, and 2) a hard fork of one chain is a hard fork of all transitively connected chains. These seem like undesirable properties to me, but it's possible I've misunderstood something. [1] http://www.blockstream.com/sidechains.pdf http://www.blockstream.com/sidechains.pdf