code: final String createHeaderFragment(String ref) {
          int size = this.propIdsAsSet.size();
          boolean sep1Needed = size > 0 && this.constraints.length > 0;
          String sep1 = sep1Needed ? ", " : "";
          String id = size > 0 ? "id=" : "";
          boolean parenNeeded = size > 0 || this.constraints.length > 0;
          String startParen = parenNeeded ? "(" : "";
          String finishParen = parenNeeded ? ")" : "";
  
          String range = rangeString();
  
          boolean sep2Needed = sep1Needed && range.length() > 0;
          String sep2 = sep2Needed ? ", " : "";
  
          return '.' + ref + startParen + id
                  + StringUtils.join(this.propIdsAsSet, ',') + sep1
                  + constraintHeaderString(this.constraints) + finishParen + sep2
                  + range;
      }
docs: Returns the default header fragment for this link.
  
  @param ref the name {@link String} of the reference of this link.
  @return a header fragment {@link String}.
  @see #headerFragment()