code: public void addNamespace(String prefix, String namespaceURI) {
          this.prefixToNsUriMap.put(prefix, namespaceURI);
          if (this.nsUriToPrefixesMap.get(namespaceURI) == null) {
              this.nsUriToPrefixesMap.put(namespaceURI, new HashSet<String>());
          }
          this.nsUriToPrefixesMap.get(namespaceURI).add(prefix);
      }
docs: Add a custom mapping from prefix to a namespace. This mapping will
  override any mappings present in this class's XML Element (if provided).
  
  @param prefix
  the namespace's prefix. Use an empty string for the
  default prefix.
  @param namespaceURI
  the namespace URI to map.