code: function customRequire(shell, name) {
    var module = shell.requiredModules[name];
    if (module) {
      return module;
    }
  
    if (_knownModules[name]) {
      module = require(name);
    }
    else if (shell.installedModules[name]) {
      // Directly load up a specified custom module from where it would have been installed
      // when using the %module command.
      var modulePath = path.join(shell.config.userPath, 'node_modules', name);
      module = require(modulePath);
    }
  
    if (module) {
      shell.requiredModules[name] = module;
    }
    else {
      throw shell.createError('Unknown module "%s". Make sure it has been installed via %module.',
                              name);
    }
  
    return module;
  }
docs: The implementation of the 'require' method that is made available within the shell. This satisfies required modules using a combination of known modules (standard and other modules available via the shell) and custom modules (previously installed using %module).