Returns the first element of a sequence.

// 1
Enumerable.create(1, 2, 3, 4)
          .first();

// THROWS EXCEPTION
Enumerable.create()
          .first();

// 22
Enumerable.create(11, 22, 33, 44)
          .first('x => x > 20');

// THROWS EXCEPTION
Enumerable.create(111, 222, 333, 444)
          .first('x => x > 444');