Composite

Composite: treat parts and wholes uniformly

Problem: you have a tree — items inside groups inside groups — and you want one operation to work on a leaf and a branch the same way.

PawWalk scenario: a walk package made of individual items and sub-packages. total_price should work whether you call it on one item or the whole nested bundle.

The trick: both a leaf and a group answer the same method. A group's answer sums its children (which may themselves be groups).