Error running concurrent process and storing results in array

[data pulverizer]

I have been using std.parallelism and that has worked quite 
nicely but it is not fully utilising all the cpu resources in my 
computation so I though it could be good to run it concurrently 
to see if I can get better performance. However I am very new to 
std.concurrency and the baby version of the code I am trying to 
run:

```
void main()
{
   import std.concurrency;
   import std.stdio: writeln;

   void process(double x, double y, long i, shared(double[]) z)
   {
     z[i] = x*y;
   }
   long n = 100;
   shared(double[]) z = new double[n];
   for(long i = 0; i < n; ++i)
   {
     spawn(&process, cast(double)(i), cast(double)(i + 1), i, z);
   }
   writeln("z: ", z);
}
```


Illicits the following error:

```
onlineapp.d(14): Error: template std.concurrency.spawn cannot 
deduce function from argument types !()(void delegate(double x, 
double y, long i, shared(double[]) z) pure nothrow @nogc @safe, 
double, double, long, shared(double[])), candidates are:
/dlang/dmd/linux/bin64/../../src/phobos/std/concurrency.d(460):   
      spawn(F, T...)(F fn, T args)
   with F = void delegate(double, double, long, shared(double[])) 
pure nothrow @nogc @safe,
        T = (double, double, long, shared(double[]))
   must satisfy the following constraint:
        isSpawnable!(F, T)
```