Mostly make sense when using this book in a formal course over this volume. The exercises all illustrate the issue of proper definitions, also formally. Anyway, we have hinted at some definitions in our formulations of the exercises. When using this book in an informal course over this volume try reformulate the definitions along the lines of: $A$ tree consists of a root and a finite set of zero, one or more subtrees. Subtrees are trees - albeit inserting root and branch labels as appropriate. We otherwise, for Exercises 6.4-6.11, refer to Sect. 6.4 , especially its subsection, Sect. 6.4 .2 , on uniqueness and identification.
Finite Branch-Labelled Trees. Define a concept of trees, informally, as in Example 6.7, but for which all branches (the things that "connect" a root of a tree with the roots of its immediate subtrees) are labelled - and such that no two branches of a tree are identically labelled.