Ported from r-project: http://code.dlang.org/packages/bessel
If you like to write the full path like "std.math.isNaN", then you can also use static imports.
Perhaps it's worth adding pre/post-conditions to the functions.The indentations probably have a mix of tabs and spaces and seem messed up.
Is gamma_cody better/worse than std.mathspecial.gamma? Bye, bearophile