code: public static Header exists(final String name)
     {
        return new Header(name, "{" + Header.class.getName() + "_value}") {
           @Override
           public String toString()
           {
              return "Header.exists(\"" + name + "\")";
           }
        };
     }
docs: Create a {@link Header} condition that matches only against the existence of a header with a name matching the
  given pattern. The header value is ignored.
  <p>
  The given name pattern may be parameterized:
  <p>
  <code>
  Header.exists("Accept-Encoding")<br>
  Header.exists("Accept-{acceptType}")<br>
  Header.exists("{anything}")<br>
  ...
  </code>
  <p>
  
  @see {@link ConfigurationRuleParameterBuilder#where(String) {@link HttpServletRequest#getHeader(String)}
  
  @param name {@link ParameterizedPattern} matching the header name.