[Zope] Behavior of <dtml-let>

I discovered an interesting attribute of <dtml-let> after it was biting me on the nose for awhile. I've got a property, "db_Columns", of type "lines." In my code, I do the following: <dtml-let columns="db_Columns"> ... <dtml-call "columns.append('blahblahblah')"> ... </dtml-let> The append works wonderfully for my "local" variable columns, but as it turns out, it also appends 'blahblahblah' to "db_Columns". I totally understand why this happens, and it certainly makes sense for <dtml-let> to function this way; I just thought that some mention of this feature in the DTML Reference guide (or some such document) could save someone else the trouble of tracking down why a screen is rendering very oddly. Thanks! -CJ