// original.d
func void StExt_SomeHandler(var int indx) { ... };
// patch.d
const int isAlt = true; //var???
func void My_SomeHandler(var int indx) { ... };
//hook
func void StExt_SomeHandler(var int indx)
{
if (isAlt) { return My_SomeHandler(indx); };
return StExt_SomeHandler_old(indx);
};