(λ (x) E) is `(function (x) E)` or `function (x) { E; }` or `\x -> E` or `lambda x: E` or `proc { |x| E }`. Everything else comes out of the incredibly clever

(λ (x) E) is `(function (x) E)` or `function (x) { E; }` or `\x -> E` or `lambda x: E` or `proc { |x| E }`. Everything else comes out of the incredibly clever realization that functions are capable of encoding everything you need in order to program.