code: @Nonnull
    public final APIDescriptor addRequiredParam (@Nullable final String sParamName)
    {
      if (StringHelper.hasText (sParamName))
        m_aRequiredParams.add (sParamName);
      return this;
    }
docs: Add a required request parameter. If this request parameter is not present,
  invocation will not be possible.
  
  @param sParamName
  The name of the required request parameter. May be <code>null</code>
  or empty in which case the parameter is ignored.
  @return this for chaining
  @see #addRequiredParams(String...)