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A law increasing the period of
limitation with respect to suits
is not impairing the obligation
of contract, even though the
debt has been barred.
But a law increasing the period
for the prosecution of a crime, if the
period has elapsed, then this is an
ex post facto law:

538: The Migratory Bird Case — Treaty — Valid.

Can a treaty be unconstitutional?
Yes. But a treaty does not have to
be in perusance of the constitution.
See: Change in wording if Article VI.
But Ribble says we have to
determine what is a treaty.

Some treaties are ipso facto law.
Others require Congress to pass laws.
So the case of Holland v. U.S. required
U.S. to pass statute toenforce the
treaty.