Just played through the game. A nice little puzzle, and definitely worth expanding on. Some more detailed thoughts (with a lot of spoilers):
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×The eye symbols were my favorite part, mostly because they're the only symbols that really use the two-dimensional layout -- all the others can be interpreted by reading left to right, top to bottom. On that note, the very first eye symbol puzzle should probably use an eye looking right -- that way, you can read the symbol as "repeat the next character" and still get the right answer. Alternatively, you could have two symbols pointing in different directions. When I could only look at that first puzzle, it wasn't really obvious that the symbol was directional at all, so it took me a little while to get the right idea.
I had a lot of trouble with the puzzles where an eye was looking at another eye. As far as I can tell, the full rule for eyes is:
-- An eye that looks at a non-eye symbol is read as that symbol
-- An eye that looks at the border is read as nothing
-- An eye that looks at another eye is read as the last non-eye symbol preceding that other eye in reading order.
That last part was very unintuitive to me. I would have expected that you should keep following the eyes until you hit a non-eye symbol, and all the eyes you've passed through are then read as that symbol. Alternatively, eyes could look straight through other eyes. But this rule, where eyes do see other eyes but are not necessarily read as the same symbol as those other eyes, is rather weird.
The negation symbol is nicely introduced, but its interaction with numbers is not really obvious. The first two puzzles where negation precedes a number had the form 4o2a and 4o2o1a (where o is the negation symbol and a is an arrow), and my first guess at reading these puzzles was that o followed by a number is essentially a syntax error, discarding everything that precedes it. This actually gives the correct answer to these two puzzles (two arrows and one arrow, respectively). Even after realizing that this is not the case, I first read o between numbers as a binary subtraction operator rather than a unary negation operator. The difference is really quite subtle -- the sequence 4*2-1a is calculated left to right (as the game establishes in the section about the interaction between multiplication and subtraction), so the result is seven arrows, but in the sequence 4*2o1a, the 2o1 part is a single group having value 1, so the result is four arrows. I think it would've been good to have a puzzle like o14a (result three arrows), reinforcing the unary nature of the o symbol, and some puzzles that focus on the interaction between negation and multiplication.
I also tripped a few times when doing the puzzles involving the repetition symbols. I think the hint is somewhat misleading -- the comparison to quotes made me think that each opening symbol would be matched to a closing symbol, and the rule is that everything between the symbols is repeated. That works at first, but then you get puzzles with extra closing symbols. My first attempt at repairing the rule was to guess that the extra closing symbols matched the start of the "sentence", so you'd still have matched pairs of symbols. It took me a while to figure out that you could have multiple closing symbols referring back to the same opening symbol -- this is quite unlike how quotes normally work. It also means that you can't have nested pairs -- my intial guess would imply that for a sequence like `a`bīcī (where ` is opening, ī is closing, and a, b and c are different arrows), the solution is abbcabbc, but the actual solution, if I've understood the rule correctly, is abbbc (because a closing symbol always refers back to the most recent opening symbol).
Mind you, I think the way the symbols work is fine, it's just that the hint doesn't quite work.
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