hence is not manifested by a single
    simple type. A small number of types work together to implement this
    concept within the library.</p>
<p>We've leveraged on the <a href="http://en.cppreference.com/w/cpp/error/error_code" target="_top">std::error_code</a>
    which is part of the standard library. We don't use all the facilities
    that it offers so it's not an exact match, but it's useful and works for
    our purposes.</p>
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