Good catch. We will have to think about it. We've usually devised exercices to have their result be achievable in the shortest amount of moves and producing the simplest form. It looks like we didn't get that one 100% right.
My 6 year old just brought this one to me, upset that he couldn't get it "right". Hope this gets fixed soon.
I've played through the entire "game" in advance of my children doing so, and this level did bother me. It marks the "negative snake" as "useless", when in reality it is part of the solution. I'm ok with some semantic debate about suboptimal solutions, but not with an integral part of the correct answer being marked as incorrect. Hopefully you guys can resolve this soon.
Still thinking about it? I just downloaded this game yesterday and this issue is still present, two and a half years later. My problem is not just that the original poster's solution is suboptimal, but the program calls the -worm card "useless".
Disappointing that you're teaching kids x = 1 + -1*worm is somehow preferable to x = 1 + -worm, and that the -worm term is "useless".
I too was upset that I couldn't get the answer. I knew that 1-worm was the answer algebraically so I couldn't get why the program kept calling it useless. It was not fun to keep trying. I assumed you had a serious bug.
Playing through to try this (as an adult), I also was confused by this level. It's the "useless" marker that was most confusing, so at first I thought the level must have a bug. I only got un-stuck by noticing that the steps were at 10/9 and trying to use fewer steps.