title: The mystery of C++20 concept boolean-testable
body: <p>C++20 introduces a comparison concept <a href="https://en.cppreference.com/w/cpp/concepts/boolean-testable" rel="noreferrer"><em>boolean-testable</em></a>, but I noticed its italics and the hyphens in the middle, indicating that it is for exposition-only, and since there is no so-called <code>std::boolean_testable</code> in <code>&lt;concepts&gt;</code>, we cannot use it in our own code.</p>
  <p>What is the purpose of this <strong>exposition-only</strong> concept? And why is this concept so mysterious?</p>