he preconditions of
  all standard library operations that it supports. Preconditions that
  are always checked (regardless of whether or not we are in debug
  mode) are checked via the <code class="code">__check_xxx</code> macros defined
  and documented in the source
  file <code class="code">include/debug/debug.h</code>. Preconditions that may or
  may not be checked, depending on the debug-mode
  macro <code class="code">_GLIBCXX_DEBUG</code>, are checked via
  the <code class="code">__requires_xxx</cod