Peel then shred the sweet potato using the coarser side of a box grater. Strain and rinse the black beans beans. Place olive oil in a large skillet and add shredded sweet potato, strained beans, salt, water, and cumin to taste. Under medium heat, cook mixture for about 10 minutes or until tender, stirring frequently so it does not burn. You may cover the skillet to keep the mixture moist. When the mixture is cooked, place a dab of butter on a clean skillet and place one flour tortilla then some shredded cheese on top to taste. Add a few spoonfuls of mixture, optionally with some warmed up leftover rice, and cook on medium high heat until the cheese is melted and the tortilla is crispy. Remove from heat and fold the tortilla. Repeat as needed.