o the number of templates directly in the
    chain, not those hidden in group templates.</p>

    <p><strong>Caveat:</strong> to chain to a base class which is
    <em>not</em> a Boost operator template when using the <a href=
    "#two_arg">single-argument form</a> of a Boost operator template, you
    must specify the operator template with the trailing <code>'1'</code> in
    its name. Otherwise the library will assume you mean to define a binary
    operation combining the class you intend to use as a 