These routines compute the dilogarithm for a real argument. In Lewin's notation this is @math{Li_2(x)}, the real part of the dilogarithm of a real @math{x}. It is defined by the integral representation @math{Li_2(x) = - \Re \int_0^x ds \log(1-s) / s}. Note that @math{\Im(Li_2(x)) = 0} for @c{$x \le 1$} @math{x <= 1}, and @math{-\pi\log(x)} for @math{x > 1}.