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

fusl/src/math/nextafter.c

Index: fusl/src/math/nextafter.c
diff --git a/fusl/src/math/nextafter.c b/fusl/src/math/nextafter.c
new file mode 100644
index 0000000000000000000000000000000000000000..ab5795a47a93e36c8c461d8a4be6621b582ae077
--- /dev/null
+++ b/fusl/src/math/nextafter.c
@@ -0,0 +1,31 @@
+#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;
+}