code: @Nonnull
      public static <T> Param createConstant (@Nonnull @Nonempty final String sParamName,
                                              @Nonnull final Class <T> aParamClass,
                                              @Nullable final T aDefault)
      {
        return new Param (sParamName, aParamClass, () -> aDefault);
      }
docs: Create a {@link Param} with a constant default value.
  
  @param sParamName
  Parameter name. May neither be <code>null</code> nor empty.
  @param aParamClass
  The parameter class. May not be <code>null</code>.
  @param aDefault
  The constant default value to be used. May be <code>null</code>.
  @return The {@link Param} object and never <code>null</code>.