__signbit

Undocumented in source but is binding to C. You might be able to learn more by searching the web for its name.
  1. int __signbit(double x)
  2. int __signbit(double )
    version(!DigitalMarsWin32 && !linux && !OSX && FreeBSD)
    extern (C)
    int
    __signbit
    (
    double
    )

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