Accepts LevelDOWN root instance

[cshum]

prefixdown(db) should accept both LevelUP and LevelDOWN instance as db

[cshum]

Wrap with levelup would do. prefixdown(levelup(down)).
Can detect and supply levelup wrapper, but that means prefixdown needs to be dependent on leveup, thus incompatibility with future levelup version.

var levelup = require('levelup')
module.exports = function (db) {
  db = db.toString() === 'LevelUP' ? db : levelup(db)
  ...
}

Would better off just require db to be levelup