code: public static RequestParameter exists(final String name)
     {
        return new RequestParameter(name, "{" + RequestParameter.class.getName() + "_" + name + "_value}") {
           @Override
           public String toString()
           {
              return "RequestParameter.exists(\"" + name + "\")";
           }
        };
     }
docs: Create a {@link Condition} that matches against the existence of a request parameter with a name matching the
  given pattern. The parameter value is ignored.
  <p>
  Parameter name expressions may be parameterized:
  <p>
  <code>
  RequestParameter.exists("username")<br/>
  RequestParameter.exists("{name}")<br/>
  ...
  </code>
  
  @param name {@link ParameterizedPattern} matching the request parameter name.
  
  @see ConfigurationRuleParameterBuilder#where(String) {@link HttpServletRequest#getParameterMap()}