docs: function transpileDerivedExchangeFiles (folder, pattern = '.js') {
  
      // exchanges.json accounts for ids included in exchanges.cfg
      const ids = require ('../exchanges.json').ids;
  
      const classNames = fs.readdirSync (folder)
          .filter (file => file.includes (pattern) && ids.includes (path.basename (file, pattern)))
          .map (file => transpileDerivedExchangeFile (folder, file))
  
      if (classNames.length === 0)
          return null
  
      let classes = {}
      classNames.forEach (({ className, baseClass }) => {
          classes[className] = baseClass
      })
  
      return classes
  }