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+--- Day 21: Allergen Assessment ---
+
+You reach the train's last stop and the closest you can get to your vacation island without getting
+wet. There aren't even any boats here, but nothing can stop you now: you build a raft. You just need
+a few days' worth of food for your journey.
+
+You don't speak the local language, so you can't read any ingredients lists. However, sometimes,
+allergens are listed in a language you do understand. You should be able to use this
+information to determine which ingredient contains which allergen and work out which foods are safe
+to take with you on your trip.
+
+You start by compiling a list of foods (your puzzle input), one food per line. Each line includes
+that food's ingredients list followed by some or all of the allergens the food
+contains.
+
+Each allergen is found in exactly one ingredient. Each ingredient contains zero or one allergen.
+Allergens aren't always marked; when they're listed (as in (contains nuts, shellfish)
+after an ingredients list), the ingredient that contains each listed allergen will be
+somewhere in the corresponding ingredients list. However, even if an allergen isn't
+listed, the ingredient that contains that allergen could still be present: maybe they forgot to
+label it, or maybe it was labeled in a language you don't know.
+
+For example, consider the following list of foods:
+
+mxmxvkd kfcds sqjhc nhms (contains dairy, fish)
+trh fvjkl sbzzf mxmxvkd (contains dairy)
+sqjhc fvjkl (contains soy)
+sqjhc mxmxvkd sbzzf (contains fish)
+
+The first food in the list has four ingredients (written in a language you don't understand):
+mxmxvkd, kfcds, sqjhc, and nhms. While the food might contain other allergens, a few allergens the
+food definitely contains are listed afterward: dairy and fish.
+
+The first step is to determine which ingredients can't possibly contain any of the
+allergens in any food in your list. In the above example, none of the ingredients kfcds, nhms,
+sbzzf, or trh can contain an allergen. Counting the number of times any of these ingredients appear
+in any ingredients list produces 5: they all appear once each except sbzzf, which
+appears twice.
+
+Determine which ingredients cannot possibly contain any of the allergens in your list. How
+many times do any of those ingredients appear?
+
+