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The goal of the library is to achieve double precision accuracy wherever
possible. However the cost of evaluating some special functions to
double precision can be significant, particularly where very high order
terms are required. In these cases a `mode`

argument allows the
accuracy of the function to be reduced in order to improve performance.
The following precision levels are available for the mode argument,

`GSL_PREC_DOUBLE`

Double-precision, a relative accuracy of approximately

*2 * 10^-16*.`GSL_PREC_SINGLE`

Single-precision, a relative accuracy of approximately

*10^-7*.`GSL_PREC_APPROX`

Approximate values, a relative accuracy of approximately

*5 * 10^-4*.

The approximate mode provides the fastest evaluation at the lowest accuracy.