The Gemara on daf zayin discussed when the bracha that is recited when performing a mitzvah takes the form "v'tzivanu l'X" versus "v'tzivanu al X". As I understood it, the explanation is that we use the "l'X" form when:

* The mitzvah is compulsory, not optional

* The mitzvah cannot be done via a shaliach

* The mitzvah must be done at a specific time

The examples in the daf included specific cases of each of these. One says "al hashchitah" rather than "lishchot" when killing an animal for food, because one has the option of eating something else. One says "al hamilah" rather than "limol" because it's usually the mohel performing the circumcision on one's son, not the father. And one says "al biur chametz" rather than "liv'or chametz" because the destruction of chametz can be done any time prior to Passover, even up to the day after Passover ended the previous year.

So here's my question. At the seder, when we fulfil the mandatory mitzvah of personally eating matzah between tzet hakochavim and chatzot on the eve of the 15th of Nisan, why do we say "al achilat matzah" and not "le'echol matzah"?

Andrew Greene, Newton, Mass, USA