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

fusl/src/math/cosh.c

+#include "libm.h"
+
+/* cosh(x) = (exp(x) + 1/exp(x))/2
+ *         = 1 + 0.5*(exp(x)-1)*(exp(x)-1)/exp(x)
+ *         = 1 + x*x/2 + o(x^4)
+ */
+double cosh(double x)
+{
+        union {double f; uint64_t i;} u = {.f = x};
+        uint32_t w;
+        double t;
+
+        /* |x| */
+        u.i &= (uint64_t)-1/2;
+        x = u.f;
+        w = u.i >> 32;
+
+        /* |x| < log(2) */
+        if (w < 0x3fe62e42) {
+                if (w < 0x3ff00000 - (26<<20)) {
+                        /* raise inexact if x!=0 */
+                        FORCE_EVAL(x + 0x1p120f);
+                        return 1;
+                }
+                t = expm1(x);
+                return 1 + t*t/(2*(1+t));
+        }
+
+        /* |x| < log(DBL_MAX) */
+        if (w < 0x40862e42) {
+                t = exp(x);
+                /* note: if x>log(0x1p26) then the 1/t is not needed */
+                return 0.5*(t + 1/t);
+        }
+
+        /* |x| > log(DBL_MAX) or nan */
+        /* note: the result is stored to handle overflow */
+        t = __expo2(x);
+        return t;
+}