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Synopsis Classify elements in a set.
Function map[&K,set[&V]] classify(set[&V] input, &K (&V) getClass)
Usage import Set;
Examples We classify animals by their number of legs. 
rascal>import Set;
ok
Create a map from animals to number of legs. 
rascal>legs = ("bird": 2, "dog": 4, "human": 2, "snake": 0, "spider": 8, "millepede": 1000, "crab": 8, "cat": 4);
map[str, int]: ("snake":0,"spider":8,"human":2,"crab":8,"cat":4,"bird":2,"dog":4,"millepede":1000)
Define function nLegs that returns the number of legs for each animal (or 0 when the animal is unknown): 
rascal>int nLegs(str animal){
>>>>>>>    return legs[animal] ? 0;
>>>>>>>}
int (str): int nLegs(str);
Now classify a set of animals: 
rascal>classify({"bird", "dog", "human", "spider", "millepede", "zebra", "crab", "cat"}, nLegs);
map[int, set[str]]: (
  0:{"zebra"},
  2:{"human","bird"},
  4:{"cat","dog"},
  8:{"spider","crab"},
  1000:{"millepede"}
)

Questions
Question [1].
Given 
import Set;
color = ("apple" : "green", "banana" : "yellow", "cucumber" : "green", "berry" : "red");
str getColor(str fruit) = color[fruit] ? "unknown";

classify({"apple", "berry", "cucumber", "banana"}, getColor) ==



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