code: public String relPlaceHolderFrom(final String rel) {
          if (isMatchingCuriedRel(rel)) {
              return rel.substring(curiedRelPrefix.length());
          }
          if (isMatchingExpandedRel(rel)) {
              return rel.substring(relPrefix.length(), rel.length() - relSuffix.length());
          }
          throw new IllegalArgumentException("Rel does not match the CURI template.");
      }
docs: Returns the placeholder from a link-relation type that is matching the {@code CuriTemplate}.
  
  <p>
  Example:
  </p>
  <pre><code>
  Link curi = Link.curi("x", "http://example.org/rels/{rel}");
  CuriTemplate template = CuriTemplate.curiTemplateFor(curi);
  
  // Returns "product":
  template.relPlaceHolderFrom("http://example.org/rels/product");
  template.relPlaceHolderFrom("x:product"):
  
  // Throws IllegalArgumentException:
  template.relPlaceHolderFrom("http://example.com/link-relations/product");
  template.relPlaceHolderFrom("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