  A C99 program is presented which calculates the Clausen sums for non-negative
integer indices j and real arguments x. The implementation is split into three
major cases. For large j, a direct, truncated summation of the sum is
performed. For smaller j, the associate polynomial of x is evaluated if
applicable; otherwise a Chebyshev series is constructed by repeated integration
of a (truncated) Chebyshev representation of Cl_2(x), and evaluated.
