_than_comparable2">less_than_comparable&lt;T,
    U&gt;</a></code>, <code><a href=
    "#equality_comparable2">equality_comparable&lt;T, U&gt;</a></code>,
    <code><a href="#equivalent2">equivalent&lt;T, U&gt;</a></code>, and
    <code><a href="#partially_ordered2">partially_ordered&lt;T,
    U&gt;</a></code>) are exceptions to this guideline, since the return type
    of the operators they define is <code>bool</code>.</p>

    <p>On compilers which do not support partial specialization, the
    two-argume