plasma-verifier
plasma-verifier is a library that makes it possible for plasma chain clients to execute predicate contracts. In a nutshell, predicate contracts are special smart contracts that run on a plasma chain. Clients need to be able to execute these contracts in order to check the validity of certain state transitions.
At its core, plasma-verifier basically just executes the EVM via ethereumjs-vm. All predicate contracts must implement a standard contract interface, so we just need the contract's bytecode in order to execute a transaction.
plasma-verifier currently only supports predicate contracts that are pure functions - meaning they don't rely on any external state. However, this is not a restriction of predicate contracts in general. It's entirely possible to build predicate contracts that do read external state by pulling the state from Ethereum and then loading it into our local EVM instance. We plan to add support for this in the near future.