I was coding the log of the Mittag-Leffler function in Stan. Everything fine, except its terrible slow.

Any ideas?

```
real mlf_log(real z, real a, real b) {
real s = -lgamma(b);
real si = s;
int k = 1;
real log_z = log(z);
if(z < 10e-15)
return s;
while(si > -35) { // precision limit exp(-35) below 6.305117e-16 stop iteration
si = k * log_z - lgamma(a * k + b);
s = log_sum_exp(s, si);
k += 1;
}
return s;
}
```