    <para>
      The relational functions <function>isequal</function>, <function>isgreater</function>,
      <function>isgreaterequal</function>, <function>isless</function>,
      <function>islessequal</function>, and <function>islessgreater</function> always return
      0 if either argument is not a number (NaN). <function>isnotequal</function> returns 1
      if one or both arguments are not a number (NaN) and the argument type is a scalar and
      returns -1 if one or both arguments are not a number (NaN) and the argument type is a vector.
    </para>
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