- It may be empty.
- It may have a root node. A node has a letter as data and refers to a left and a right subtree. The left and right subtrees are also binary trees of letters.

- Empty trees are omitted completely.
- Each node is indicated by
- Its letter data,
- A line segment down to the left to the left subtree, if the left subtree is nonempty,
- A line segment down to the right to the right subtree, if the right subtree is nonempty.

- If the tree is empty, then the preorder traversal is empty.
- If the tree is not empty, then the preorder traversal consists of the following, in order
- The data from the root node,
- The preorder traversal of the root's left subtree,
- The preorder traversal of the root's right subtree.

The input will contain one or more data sets. Each data set is a sequence of one or more lines of capital letters.
The lines contain the leaves removed from a binary search tree in the stages described above. The letters on a line will be listed in increasing alphabetical order. Data sets are separated by a line containing only an asterisk ('*').
The last data set is followed by a line containing only a dollar sign ('$'). There are no blanks or empty lines in the input.

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