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