It has a single root and each node has 0..N ordered sub-nodes . The keys represent a distinct set of paths. Two trees can only be merged if they share a common root. It needs to support, at minimum: insert, merge, enumerate paths.

For this tree:

The
 +-------+----------------+
 |       |                |
cat     cow              dog
 +       +--------+       +
 |       |        |       |
drinks  jumps    moos    barks
 +
 |
milk

the paths would be:

• The cat drinks milk
• The cow jumps
• The cow moos
• The dog barks

It's a bit like a trie. What is it?