docs: function createLogger (writables, prefix, dateFormat) {
    if (!Array.isArray(writables)) writables = [ writables ]
  
    writables = writables.filter(w => isDefined(w) && isFunction(w.write))
  
    debug(`There are ${writables.length} writables for this logger`)
  
    let pre = `[${prefix}]`
    // default timestamp format
    if (dateFormat === true) dateFormat = 'YYYY-MM-DD HH:mm:ss'
    if (typeof dateFormat === 'string') {
      pre += ` ${moment().format(dateFormat)}`
    }
  
    return msg => {
      writables.forEach(w => w.write(`${pre} ${msg}\n`))
    }
  }