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I'm not psyched about the patch.  First, I've always thought
that case insensitivity ought to be handled by the mapping
from which the keys are being extracted and by setting the
idpattern.  Second, I definitely don't like adding the
case_sensitive argument to substitute() and
safe_substitute(). Because it lives in the same namespace as
kws it makes for icky rules on resolving any conflicts.

Is there a reason why the existing mechanisms are insufficient?