code: @PublicEvolving
  	public boolean contains(ConfigOption<?> configOption) {
  		synchronized (this.confData){
  			// first try the current key
  			if (this.confData.containsKey(configOption.key())) {
  				return true;
  			}
  			else if (configOption.hasFallbackKeys()) {
  				// try the fallback keys
  				for (FallbackKey fallbackKey : configOption.fallbackKeys()) {
  					if (this.confData.containsKey(fallbackKey.getKey())) {
  						loggingFallback(fallbackKey, configOption);
  						return true;
  					}
  				}
  			}
  
  			return false;
  		}
  	}
docs: Checks whether there is an entry for the given config option.
  
  @param configOption The configuration option
  
  @return <tt>true</tt> if a valid (current or deprecated) key of the config option is stored,
  <tt>false</tt> otherwise