code: function (block, buffer, offset) {
      var start = offset % block
      var length = buffer.readUInt16LE(start)
      if(length === block - 1)
        return null
      else {
        var _length = buffer.readUInt16LE(start+2+length)
        if(_length != length)
          throw new Error('expected matching length at end, expected:'+length+', was:'+_length)
        result.offset = offset
        result.start = start+2
        result.length = length
        return result
      }
    }
docs: getRecord(buffer, 0) for first record in block