Question: How do beans and rice contribute to a good diet for vegetarians?
About the article "Cereal", section "Nutritional facts".

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Some grains are deficient in the essential amino acid lysine.
That is why many vegetarian cultures, in order to get a balanced diet, combine their diet of grains with legumes.
Many legumes, on the other hand, are deficient in the essential amino acid methionine, which grains contain.
Thus, a combination of legumes with grains forms a well-balanced diet for vegetarians.
Common examples of such combinations are dal (lentils) with rice by South Indians and Bengalis, dal with wheat in Pakistan and North India, and beans with corn tortillas, tofu with rice, and peanut butter with wheat bread (as sandwiches) in several other cultures, including Americans.
The amount of crude protein found in grain is measured as the grain crude protein concentration.
"Beans, Beans, The Musical Fruit" is a schoolyard saying and children's song about the capacity for beans to contribute to flatulence.
The song is also variously known as "Beans, Beans, the Magical Fruit", "Beans, Beans, the Miracle Fruit", and "Beans, Beans, the Wonderful Fruit".
One variation of the song (sometimes called the second verse) is titled "Beans, Beans, They're Good for Your Heart.
"The basis of the song (and bean/fart humor in general) is the high amount of oligosaccharides present in beans.
Bacteria in the large intestine digest these sugars, producing carbon dioxide and hydrogen.
These gases are expelled from the body as flatulence. 
Despite a widespread belief that vegetarians must eat grains and beans within a few hours of each other to make a 'complete' protein which contains all 9 "essential amino acids", this has never been substantiated by research.
The protein-combining theory was brought to popular attention in Frances Moore Lappé's 1971 bestseller "Diet for a Small Planet".
In later editions of the book, as early as 1981, Lappé withdrew her contention that protein 'combining' is necessary. 
  "The history and chemistry of neoarsphenamine" (1921)  "Preservation of Philippine foods" (1926)  "Rice bran: a health food and how to cook it" (1932)  "Roselle recipes" (1931)  "Soy beans as a component of a balanced diet and how to prepare them" (1932)  
Vegetarian diets typically contain similar levels of iron to non-vegetarian diets, but this has lower bioavailability than iron from meat sources, and its absorption can sometimes be inhibited by other dietary constituents.
According to the Vegetarian Resource Group, consuming food that contains vitamin C, such as citrus fruit or juices, tomatoes, or broccoli, is a good way to increase the amount of iron absorbed at a meal.
Vegetarian foods rich in iron include black beans, cashews, hempseed, kidney beans, broccoli, lentils, oatmeal, raisins, spinach, cabbage, lettuce, black-eyed peas, soybeans, many breakfast cereals, sunflower seeds, chickpeas, tomato juice, tempeh, molasses, thyme, and whole-wheat bread.
The related vegan diets can often be higher in iron than vegetarian diets, because dairy products are low in iron.
Iron stores often tend to be lower in vegetarians than non-vegetarians, and a few small studies report very high rates of iron deficiency (up to 40%, and 58% of the respective vegetarian or vegan groups).
However, the American Dietetic Association states that iron deficiency is no more common in vegetarians than non-vegetarians (adult males are rarely iron deficient); iron deficiency anaemia is rare no matter the diet.