code: function runBlock (cb) {
        self.runBlock({
          block: block,
          root: parentState
        }, function (err, results) {
          if (err) {
            // remove invalid block
            blockchain.delBlock(block.header.hash(), function () {
              cb(err)
            })
          } else {
            // set as new head block
            headBlock = block
            cb()
          }
        })
      }
docs: run block, update head if valid