Better proposition tester than "is"?



Hallo.

I'm sorry I'm asking such a simple question, but is there a better
proposition tester than is()? The function is() is really poor. Let us
assume this case:

assume(x>0,x<1,y>0,y<1);
is(x>0);
is(x+y>0);
is(x*y>0);
is(x+y<2);
is(x*y<1);

The first three questions are correctly answered true, but the last
two questions are unknown. Is there a (automatic) way around such
obvious problems?

Many thanks,
Michal