code: function removeAll (state, prefix, filter) {
    var docs
  
    var options = {
      include_docs: true
    }
  
    if (prefix) {
      options.startkey = prefix
      options.endkey = prefix + '\uffff'
    }
  
    return state.db.allDocs(options)
  
    .then(function (res) {
      docs = res.rows
        .filter(isntDesignDoc)
        .map(function (row) {
          return row.doc
        })
  
      if (typeof filter === 'function') {
        docs = docs.filter(filter)
      }
  
      return docs.map(function (doc) {
        doc._deleted = true
        return addTimestamps(doc)
      })
    })
  
    .then(bulkDocs.bind(null, state))
  }
docs: removes all existing docs
  
  @param  {String}   prefix     optional id prefix
  @param  {Function} [filter]   Function returning `true` for any doc
  to be removed.
  @return {Promise}