code: protected Object unknownVariable(JexlException xjexl) {
  		// don't throw the exception
  		if (!silent) {
  			logger.trace(xjexl.getMessage());
  		}
  		return null;
  	}
docs: Triggered when variable can not be resolved.
  
  @param xjexl
  the JexlException ("undefined variable " + variable)
  @return throws JexlException if strict, null otherwise