I made a slight error in the definition of observational congruence; the
corrected version is
DEF: Two terms M and N are *observationally congruent* if for
any closing context C[ ] with C[M], C[N] of integer type,
Eval(C[M]) and Eval(C[N]) either both do not halt or one
equals the numeral 0 iff the other equals 0.
-Jon Riecke