examples/run-blockchain/index.js
function getHead (next) {
    vm.blockchain.getHead(function (err, block) {
      // make sure the state is set before checking post conditions
      state.root = block.header.stateRoot
      next(err)
    })
  }
function getHead (next) {
    vm.blockchain.getHead(function (err, block) {
      // make sure the state is set before checking post conditions
      state.root = block.header.stateRoot
      next(err)
    })
  }
get the blockchain head