code: public NamespaceHeader findNamespaceHeader(final String rloc) {
          if (noLength(rloc)) {
              throw new InvalidArgument("rloc", rloc);
          }
  
          return headers.get(rloc);
      }
docs: Finds the {@link NamespaceHeader namespace header} for the namespace's
  {@link String resource location}.
  
  @param rloc namespace {@link String resource location}, which cannot be
  {@code null}
  @return {@link NamespaceHeader namespace header} or {@code null} if one
  is not found
  @throws InvalidArgument Thrown if {@code rloc} is {@code null} or empty