cpp/numeric/special functions/expint

@1-3@ Computes the of. @A@ Additional overloads are provided for all integer types, which are treated as.

Return value
If no errors occur, value of the exponential integral of, that is $-$, is returned.

Error handling
Errors may be reported as specified in math_errhandling.
 * If the argument is NaN, NaN is returned and domain error is not reported.
 * If the argument is ±0, -∞ is returned.