Add a "fork" of musl as //fusl.

fusl/src/math/nextafter.c

+#include "libm.h"
+
+double nextafter(double x, double y)
+{
+        union {double f; uint64_t i;} ux={x}, uy={y};
+        uint64_t ax, ay;
+        int e;
+
+        if (isnan(x) || isnan(y))
+                return x + y;
+        if (ux.i == uy.i)
+                return y;
+        ax = ux.i & -1ULL/2;
+        ay = uy.i & -1ULL/2;
+        if (ax == 0) {
+                if (ay == 0)
+                        return y;
+                ux.i = (uy.i & 1ULL<<63) | 1;
+        } else if (ax > ay || ((ux.i ^ uy.i) & 1ULL<<63))
+                ux.i--;
+        else
+                ux.i++;
+        e = ux.i >> 52 & 0x7ff;
+        /* raise overflow if ux.f is infinite and x is finite */
+        if (e == 0x7ff)
+                FORCE_EVAL(x+x);
+        /* raise underflow if ux.f is subnormal or zero */
+        if (e == 0)
+                FORCE_EVAL(x*x + ux.f*ux.f);
+        return ux.f;
+}