I was using `scipy.stats.wald.fit` to fit my data array and, without any warnings or errors, I got two parameters (-1.004123, 0.11517). I plot the distribution plot and the histogram plot and found them match well. 

However, there are negative values in my data and wald distribution should only work for positive values, <http://en.wikipedia.org/wiki/Inverse_Gaussian_distribution>. How this `scipy.stats.wald.fit` works? Can anyone give me the mathematical expression of the `scipy.stats.wald.pdf`?

I actually would like to write the wald pdf function in Cuda-C, so a mathematical expression is needed.