Write a function in pseudocode to calculate the number of leaf nodes of a binary tree. A node in a binary tree has two pointers, named 'left' and 'right', where the 'left' pointer is used to find the left child of the node and the 'right' pointer is used to find the right child. Your function will take a node as the input and return an integer representing the number of leaf nodes in the tree rooted at that node.