+/*
+ Some notes about the structure of a symmetry tree:
+ - The leaf nodes of the tree are of type "none", have one index, and no
+ children (of course). They are constructed by the symmetry(unsigned)
+ constructor.
+ - Leaf nodes are the only nodes that only have one index.
+ - Container nodes contain two or more children. The "indices" set member
+ is the set union of the index sets of all children, and the "children"
+ vector stores the children themselves.
+ - The index set of each child of a "symm", "anti" or "cycl" node must
+ have the same size. It follows that the children of such a node are
+ either all leaf nodes, or all container nodes with two or more indices.
+*/
+