code: function Executable(log, command) {
    /** @type {string} */
    this.command = command
  
    /** @type {Array.<function(Error, Buffer, Buffer)>} */
    this.callbacks = []
  
    /** @type {Arguments} */
    this.results = null
  
    var startTime = Date.now()
    child_process.exec(this.command, function() {
      var timeDiff = `${Date.now() - startTime}`
      if (timeDiff.length < 3) timeDiff = `   ${timeDiff}`.slice(-3)
      log.debug(`Exec | ${timeDiff}ms | ${this.command}`)
      this.results = arguments
  
      for (var i = 0; i < this.callbacks.length; i++) {
        var callback = this.callbacks[i]
        callback.apply(null, arguments)
      }
    }.bind(this))
  }
docs: A memoized executable command. The first time the command is run, the results are stored.
  Callbacks are tracked while the command is running to avoid race conditions.
  
  @constructor
  @param {Log} log A logging instance.
  @param {string} command The command to execute.