What would "pencas de maguey asados" be? Is some part of the maguey
plant commonly eaten in Mexico, or does this part of the recipe just
get used and then thrown away? I'm trying to imagine whether it's
edible or adds flavor, or is superfluous.

Barbacoa (Barbecued Lamb)
(para 20 a 24 personas) (for 20 to 24 people)
1 pierna de cordero de 6 a 8 libras (1 leg of lamb, about 6 to 8
pounds)
4 libras de costillas de cordero (4 pounds of lamb ribs)
1/2 libra de garbanzos previamente remojados (1/4 pound of previously
soaked garbanzos)
2 libras de zanahorias cortadas en cuadritos (2 pounds of diced
carrots)
6 hojas de laurel (6 bay leaves)
10 dientes de ajo (10 cloves of garlic)
10 tomates verdes con cascara partidos in cuatro (10 green tomatoes
with skin, cut into quarters)
4 litros de pulque, cerveza or agua (4 liters of pulque, beer, or
water)
8 pencas de maquey asadas (8 pencas of roasted maguey)
12 hojas de aguacate (12 avocado leaves)
6 chiles verdes (opcional) (6 green chiles--optional)

Preparacion: (Preparation

En el fondo del recipinete pone el liquido, los garbanzos, zanahorias,
laurel, ajo, chiles, 4 hojas de aguacate.

(Place the liquid, the garbanzos, carrots, bay leaf, garlic, chile 4
avocado leaves in the bottom of the steamer).

Se coloca la rejilla en el parte superior de la vaporera, que se forra
con pencas de maguey; se colocan las carnes alternando con hojas de
aguacate, se cubre con mas pencas de maguey y se tapa sellando la
orilla del recipiente con masa de tortillas o una mezcla de harina y
agua que forme una masa parecid a migaja de pan.

(Place the grid in the upper part of the steamer , so that it is
covered with pencas of maguey; the meats are placed alternating with
avocado leaves, it is covered with more pencas of maguey and is covered
sealing to the border of the container with tortilla masa or a mixture
of flour and water that forms a crust similar to breadcrumbs.)

Se cuece directamente al fuego durante 4 hours aproximadente.

(Cook directly over the fire for approximately 4 hours.)