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There is an infix operator for testing membership, ><. The >< operator tests the number or string in the right operand for membership in the map or array given by the left operand. For maps, membership extends to keys only. 

a = [5, 6, 7];
m = {"a" : 1, "b" : 2};
a >< 6     // returns true
a >< 3     // returns false
m >< "a"   // returns true
m >< "foo" // returns false

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