code: public String expandedRelFrom(final String rel) {
          if (isMatchingCuriedRel(rel)) {
              return curi.getHrefAsTemplate().set("rel", relPlaceHolderFrom(rel)).expand();
          }
          if (isMatchingExpandedRel(rel)) {
              return rel;
          }
          throw new IllegalArgumentException("Rel does not match the CURI template.");
      }
docs: Returns the {@code rel} in expanded format.
  
  <p>
  Example:
  </p>
  <pre><code>
  Link curi = Link.curi("x", "http://example.org/rels/{rel}");
  CuriTemplate template = CuriTemplate.curiTemplateFor(curi);
  
  // Returns "http://example.org/rels/product":
  template.curiedRelFrom("http://example.org/rels/product");
  template.curiedRelFrom("x:product"):
  
  // Throws IllegalArgumentException:
  template.curiedRelFrom("http://example.com/link-relations/product");
  template.curiedRelFrom("y:product");
  </code></pre>
  
  @param rel a link-relation type.
  @return boolean
  @throws IllegalArgumentException if the {@code rel} parameter does not match the {@code CuriTemplate}.
  @since 2.0.0