Cannot iterate over Derivation nodes except (pre)terminals

[goodmami]

The UDFNode.terminals() and UDFNode.preterminals() methods recursively descend through the tree and return lists of nodes, but for all other nodes the user must do this descent themselves. In addition to iterating over nonterminals (or non-preterminals), a way to get the root node (if it exists) could be handy. Currently one must test if node.is_root() is True.

Calling everything above preterminals "nonterminals" wouldn't be quite right, as preterminals are nonterminals, too. The wiki uses "internal nodes", so maybe internals()?

Whatever it's called, the desired change in functionality would be something like:

• Derivation.root() -- return the root node if there is one, otherwise None
• UDFNode.internals() (or something) -- return all non-root nodes above the preterminals

As for the iteration order, depth-first makes sense as it's the order encountered in the UDF string.

[goodmami]

Let's go with internals() for the node-listing function, and let's drop the idea of the root() method until a real need arises.