MB 12: is exempt - From reading the Shema and the Amidah and from performing all [positive] Commandments [see the very first Bi'ur Halacha on this Siman --SP] because he is engaged in performing a Mitzvah, namely he is guarding the deceased from rats (****); and even on a ship we are afraid of rats [attacking the deceased] (Gemarah). He is not even permitted to be strict on himself [and read the Shema], even if he is sitting more than 4 Amos [cubits - approximately 2 metres] (*****) from the deceased (Magen Giborim), and see what we wrote above in Siman 38.
[(****) When a person dies, it is incumbent on his or her relatives (or if none are available, then the community, usually the "Chevra Shemira") to guard the body of the deceased as (originally, at least) they were afraid that rats may come and attack and eat it. There is also the concern that no-one should steal the body. Nowadays, even when such concerns may be no longer applicable (namy bodies are kept in mortuaries, we still have the custom to guard the body. --SP]
[(*****) A person's "domain" (for various purposes, including for example the laws concerning carrying on Shabbos where we say that a person may carry within his own "domain") extends for 4 Amos in each direction. So if the person guarding the deceased is beyond this, one might say that as he is outside the deceased's "domain" (almost as if he were in another room) he is permitted to read the Shema, etc.. The Mishnah Berurah, therefore, comes to tell us that this is not so. --SP]
72:4. If two people were guarding the deceased, one of them may read [the Shema] while the other is guarding and then the other may read [the Shema] while the first one guards.
Stephen Phillips stephenp@cix.compulink.co.uk
Siman 72. The Laws regarding those who carry the coffin, the comforters and those who accompany the deceased
72:4. Once they have buried the deceased, and the mourners turned to accept comfort and the people followed them from the graveyard to the place where the mourners stand to make a row to accept comfort - if the people can start and complete even one verse before they reach the row (12) they should start but if not they should not start. {Rama: If (13) there is still time to read the Shema later}. [There used to be a custom for mourners to stand in a row, and accept comfort from all others who had attended while the latter were leaving. The others would form a row of their own, in order to offer comfort one by one. -- YM]
MB 12: They should start - and read whatever they can.
MB 13: There is still time - but if they will miss the time for the Shema they should read the Shema first, because they have not yet started the Mitzvah of comforting the mourners.
72:5. Those who stand in the row to comfort: those who stand on the inside facing the mourners are exempt [from saying the Shema], but those that stand on the outside not facing the mourners are obliged [to say the Shema].