code: public function set($key, $value)
      {
          if ($this->exists($key)) {
              $this->dataStore[$key] = $value;
          }
      }
docs: Set the given value to the given key.
  
  The given key should match one of the parameters about, but with out the
  prefix. That is added on during this process.
  
  @param string $key
  @param mixed $value