code: private Constant parseConstant(Node el, boolean ignoreCase) {
          return new Constant(
                  nodeAttribute(el, TAG_CONSTANT_ATTR_VALUE),
                  nodeAttribute(el, TAG_CONSTANT_ATTR_IGNORE_CASE, ignoreCase),
                  nodeAttribute(el, TAG_ATTR_GHOST, PatternElement.DEFAULT_GHOST_VALUE)
          );
      }
docs: Parse constant pattern element
  
  @param el         DOM element
  @param ignoreCase if true the constant must ignore case
  @return constant definition