non-monotonicity

IBIS experts/users:
I have a question regarding monotonicity of IBIS model curves. I have
encountered some logic IBIS models (clock drivers) which pass the golden
parser (with some warnings), but do not pass the IBIS parser within my
simulator. The error I get is
'the sum of the maximum clamp curves and the pullup curve is non-monotonic
at -0.5 volts'.
I know for a fact that another simulator we have in house parsed these
models. My question is, do some devices naturally have non-monotonic
curves, and should an IBIS simulator accept this as a standard condition?
Does anyone have experience modifying IBIS models to remove the
non-monotonicity?

Thank you very much,
   Fabrizio Zanella
   Hardware Engineering
   email: fzanella@emc.com
  (508) 435-1000 Ext. 4645
   Fax: (508) 435-8949
   EMC Corporation
   171 South Street
  Hopkinton, MA 01748-9103
Received on Thu Aug 6 14:09:42 1998