code: function use(value) {
      var settings
  
      assertUnfrozen('use', frozen)
  
      if (value === null || value === undefined) {
        // Empty.
      } else if (typeof value === 'function') {
        addPlugin.apply(null, arguments)
      } else if (typeof value === 'object') {
        if ('length' in value) {
          addList(value)
        } else {
          addPreset(value)
        }
      } else {
        throw new Error('Expected usable value, not `' + value + '`')
      }
  
      if (settings) {
        namespace.settings = extend(namespace.settings || {}, settings)
      }
  
      return processor
  
      function addPreset(result) {
        addList(result.plugins)
  
        if (result.settings) {
          settings = extend(settings || {}, result.settings)
        }
      }
  
      function add(value) {
        if (typeof value === 'function') {
          addPlugin(value)
        } else if (typeof value === 'object') {
          if ('length' in value) {
            addPlugin.apply(null, value)
          } else {
            addPreset(value)
          }
        } else {
          throw new Error('Expected usable value, not `' + value + '`')
        }
      }
  
      function addList(plugins) {
        var length
        var index
  
        if (plugins === null || plugins === undefined) {
          // Empty.
        } else if (typeof plugins === 'object' && 'length' in plugins) {
          length = plugins.length
          index = -1
  
          while (++index < length) {
            add(plugins[index])
          }
        } else {
          throw new Error('Expected a list of plugins, not `' + plugins + '`')
        }
      }
  
      function addPlugin(plugin, value) {
        var entry = find(plugin)
  
        if (entry) {
          if (plain(entry[1]) && plain(value)) {
            value = extend(entry[1], value)
          }
  
          entry[1] = value
        } else {
          attachers.push(slice.call(arguments))
        }
      }
    }
docs: Plugin management.  Pass it: *   an attacher and options, *   a preset, *   a list of presets, attachers, and arguments (list of attachers and options).