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    Really enjoyed this kata!

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    Found it! \o/

    For future solvers, I found a collection of whitespace programs here. Compiling and tracing those helped me figure out what was going on.

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    Ack! I'm almost done except for "Testing conditional and unconditional jump functionality", tests 5 and 6:

    '3' should equal '321'
    '3' should equal '3210'
    

    I don't know what the actual tests are, but I'm able to write tests that should loop from 3 down to 1 or 0, and they pass:

    loop321 = bleach(
        ''.join([
            'ss', 'sttn',          # PUSH 3
            'nss', 'stssssttn'     # MARK "C"
            'sns',                 # DUPLICATE
            'tnst',                # OUTPUTN
            'ss', 'stn',           # PUSH 1
            'tsst',                # SUB
            'sns',                 # DUPL
            'nts', 'stssststn',    # IFZERO "E"
            'nsn', 'stssssttn',    # GOTO "C"
            'nss', 'stssststn',    # MARK "E"
            'snn',                 # DISCARD
            'nnn'
        ])
    )
    Test.assert_equals(whitespace(loop321),'321')
    loop3210 = bleach(
        ''.join([
            'ss', 'sttn',          # PUSH 3
            'nss', 'stssssttn'     # MARK "C"
            'sns',                 # DUPLICATE
            'tnst',                # OUTPUTN
            'ss', 'stn',           # PUSH 1
            'tsst',                # SUB
            'sns',                 # DUPL
            'ntt', 'stssststn',    # IFNEG "E"
            'nsn', 'stssssttn',    # GOTO "C"
            'nss', 'stssststn',    # MARK "E"
            'snn',                 # DISCARD
            'nnn'
        ])
    )
    Test.assert_equals(whitespace(loop3210),'3210')
    

    Any clues for how to diagnose this case further?

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    This was a lot of fun! I haven't written an interpreter in a long time. I found Ruslan Spivak's series of articles to be very helpful.

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    This comment is hidden because it contains spoiler information about the solution

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    I had trouble passing the exception raising test for an invalid rank. I kept catching it at the place it causes trouble and raising it up to the calling function, but that wouldn't satisfy the test suite. Instead, I had to manually check that the rank was valid within inc_progress.

    I think test.expect_error should be more tolerant and allow errors to be raised from anywhere in the call stack.

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    Nice job with the docstring

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    Wow, this kata took me from binary long division to DFAs to Arden's Theorem. It was very tempting to google for a solution but I'm glad I didn't.

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    This is O(n^2) in time isn't it? It requires computing all n+1 Fibonacci numbers. But given andreidemin's comment, you only need one Fibonacci number.

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    The most clever would be to mathemtically find the equation for nth term in the sequence.

    Interesting idea. The general solution to this 3rd order linear difference equation is a
    linear combination of exponentials r^n, where r is a solution to x^3-x^2-x-1 = 0. There are
    three roots r, two of which are nonreal. To find the coefficients, a 3x3 system of linear
    equations needs to be solved.