chametz that was bitul b'rov before pesach, is mutar on pesach. How did it become botul? Before pesach it is heter b'heter!
Mendy Kaplowitz
There are Mekoros which state that Heter b'Heter is not Batel. However, there also are many Mekoros brought by the Rishonim to indicate that Heter b'Heter is Batel as long as the mixture is inseparable. One example is the Mishnah in Kilayim (9:1) regarding a mixture of camel wool, sheep wool and linen not being Kilayim if most of the mixture is from camel wool (which even concerns a Halachah mi'd'Oraisa). Basar b'Chalav is also an example (though that can be deemed a Chidush).
The Or Zarua (1:779) brings many other examples, and Paskens like the Rivash that Heter b'Heter is Batel. [Logically, there does not seem to be any problem saying Heter b'Heter is Batel, as one can merely say that Batel refers to a small amount of something which is mixed beyond recognition into something else.]
All the best,
Yaakov Montrose