> transporting class ought to look like:</p>

<dl>
<dt><b>1. Lightweight</b></dt>
<dd>A simple-as-possible <code>T</code> and/or <code>E</code> transport without any
implementation complexity.</dd>
<dt><b>2. Medium</b></dt>
<dd>A variant stored <code>T</code> or <code>E1</code> ... <code>E<i>n</i></code>
where <code>T</code> is the expected value and <code>E1 ...</code>
are the potential unexpected values. This implemention really ought to be implemented
using C++ 17's <code>std::variant<...></code> except 