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    const flatMap = f => {
      if (!f) {
        throw new TypeError('f must be a function');
      }

      return xs => ({
        [Symbol.iterator]: function * () {
          for (const x of xs) {
            yield * f(x);
          }
        }
      });
    };


    // Usage
    const xs = [ 1, 2, 3 ] |> flatMap(x => [ x, -x ]);

    const map = f => xs => ({
      [Symbol.iterator]: function * () {
        for (const x of xs) {
          yield f(x);
        }
      }
    });

    xs |> map(x => x * 2);