 a group of operators, use the
    <code>B</code> parameter to chain operator templates into a single-base
    class hierarchy, demostrated in the <a href="#example">usage example</a>.
    The technique is also used by the composite operator templates to group
    operator definitions. If a chain becomes too long for the compiler to
    support, try replacing some of the operator templates with a single
    grouped operator template that chains the old templates together; the
    length limit only applies t