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

fusl/src/complex/casin.c

Index: fusl/src/complex/casin.c
diff --git a/fusl/src/complex/casin.c b/fusl/src/complex/casin.c
new file mode 100644
index 0000000000000000000000000000000000000000..dfdda988bafcda9403b7e03ef55f4a0eaf9349a9
--- /dev/null
+++ b/fusl/src/complex/casin.c
@@ -0,0 +1,16 @@
+#include "libm.h"
+
+// FIXME
+
+/* asin(z) = -i log(i z + sqrt(1 - z*z)) */
+
+double complex casin(double complex z)
+{
+	double complex w;
+	double x, y;
+
+	x = creal(z);
+	y = cimag(z);
+	w = CMPLX(1.0 - (x - y)*(x + y), -2.0*x*y);
+	return clog(CMPLX(-y, x) + csqrt(w));
+}