code: public function add($name, $value)
      {
          if (array_key_exists($name, $this->storage)) {
              throw new AlreadyDefinedException(sprintf('Context value with key `%s` already defined', $name));
          }
  
          $this->storage[$name] = $value;
      }
docs: Add a context value. The key must be unique and cannot be replaced
  
  @param string $name
  @param mixed $value
  
  @return void
  @throws AlreadyDefinedException