code: public void setNameSpace(String namespaceURL, String prefix)
              throws IllegalArgumentException {
          Collection<String> s = namespace.get(namespaceURL);
  
          if (s == null) {
              s = new HashSet<String>();
          }
  
          s.add(prefix);
          namespace.put(namespaceURL, s);
          this.prefixes.put(prefix, namespaceURL);
      }
docs: <p>Set or add a namespace to the context and associated it with a prefix.
  </p><p>
  A given prefix can only be associated with one namespace in the context.
  A namespace can have multiple prefixes.
  </p>
  The prefixes: {@code xml}, and {@code xmlns} are reserved and
  predefined in any context.
  
  @param namespaceURL the namespace uri
  @param prefix       the prefix to register with the uri
  @throws IllegalArgumentException thrown when trying to assign a
  namespace to a reserved prefix