![The gradient of the bivariate normal cumulative distribution](https://blogs.sas.com/content/iml/files/2013/09/gradbivariatecdf.png)
I'm spoiled by the internet. I've grown so accustomed to being able to instantly find an answer to any query—no matter how obscure—that I am surprised when I don't find what I am looking for. The other day I was trying to find a mathematical result: a formula for the