Good question. That part of the article could probably use another diagram to explain it.
The resulting document is generated by doing a depth-first prefix traversal of the tree. The ambiguity comes because "b" and "X" are both direct children of "a". So its not clear how they should be ordered relative to each other. Because "c" is a child of "b" in this example, the "X" can't appear between the "c" and "b". The only valid orderings are, as I said, "aXbc" or "abcX". But without knowing how "b" and "X" should be ordered, its ambiguous which one to use.
Let me know if thats still confusing! This stuff is hard to explain without a whiteboard.
The resulting document is generated by doing a depth-first prefix traversal of the tree. The ambiguity comes because "b" and "X" are both direct children of "a". So its not clear how they should be ordered relative to each other. Because "c" is a child of "b" in this example, the "X" can't appear between the "c" and "b". The only valid orderings are, as I said, "aXbc" or "abcX". But without knowing how "b" and "X" should be ordered, its ambiguous which one to use.
Let me know if thats still confusing! This stuff is hard to explain without a whiteboard.