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The docs should spell-out that the safe in safe_substitute()
means that no exceptions will be raised except possible
memory errors.  In some sense, it is now anything but safe.
 It will silently skip past dangling delimiters, unclosed
braced identifiers such as ${oops, identifiers with
incorrect bracing such $<angled>, and idenfiers that aren't
valid python identifiers such as $1two3.