To implement the no-init arity of reduce, you have to use the first element of the sequence as an init value, and then you have to skip the first element when you iterate through the sequence. The tests in test-reduce, which focus on using reduce to sum up a bunch of numbers in a range, don't actually pin down this behavior, because the range used starts with zero, and an extra zero doesn't affect the sum.

Screened by: Alex Miller - this is a good tweak to get better tests for the hard this easy to mess up reduce behavior.