How do you calculate square root with nodes?

I'm creating my own merge functions in the compositor, but I've run into a bit of a wall. How do you calculate the square root with nodes? Would one have to write a python node to do this? If so, what would that look like?

• To generalize a bit, the nth root is equal to the inverse power, i.e. root(x, n) == x^(1/n). So if you wanted, you could construct a node group which divides 1 by the root before feeding it to a power node, and then you'd have a root node.