code: function classify (s) {
    const keys = {
      ak: 'account_pubkey',
      ok: 'oracle_pubkey'
    }
  
    if (!s.match(/^[a-z]{2}_.+/)) {
      throw Error('Not a valid hash')
    }
  
    const klass = s.substr(0, 2)
    if (klass in keys) {
      return keys[klass]
    } else {
      throw Error(`Unknown class ${klass}`)
    }
  }
docs: What kind of a hash is this? If it begins with 'ak_' it is an
  account key, if with 'ok_' it's an oracle key.
  
  @param s - the hash.
  returns the type, or throws an exception if type not found.