### Binary Tree Count Children Sum to Even

Create a method named `countChildSumToEven`

that
accepts a single `BinaryTree<Int>?`

and counts the number of nodes in the tree where the values of the two
children sum to an even number.
You should not count nodes that don't have at least one child.
Keep in mind that not every node has a right or left child, so you'll need to check for `null`

carefully.
However, you can assume that all the *values* in the tree are `Int`

s.

For reference, `cs125.trees.BinaryTree`

is defined like this: