An Overriding 'Pathagorean' Principle
You should never 'assemble' any documents using an original document(s) as the 'base.' You should only work with copies of the original text. So long as you abide by this rule, you will not accidentally overwrite the original by forgetting to do a 'SaveAs'.
When used as suggested, Pathagoras automatically implements this rule for you. When you select a document using Pathagoras tools, you will always be working on a copy.
How do you know it is a copy? Look for the 'name' of the document in the upper left hand corner of the screen. It will be named "Document1", "Document5", etc.
Of course, there is one exception to the above rule. When you are intentionally editing the original with the goal of improving the source text itself (refining the text, adding variables, correcting spelling errors, etc.), you must open and work on the original document. But NEVER NEVER NEVER use the original of a document with the idea of editing it to become a final document for a specific client or customer.
If you find yourself working on the original for other than source editing purposes, you should rethink your process, and implement the tools provided by the program. If you do nothing else in your early days with the program you should at least "Create your First Library" and "Shelve your First Book"