Probably not much of a backstory, it's common to make copy-and-paste mistakes when implementing multiple similar things. For instance, when implementing both th

Probably not much of a backstory, it's common to make copy-and-paste mistakes when implementing multiple similar things. For instance, when implementing both the Add and Sub traits, it would be normal to start with one of them, and duplicate it to make the other, making the relevant changes in the copy (Add -> Sub, add -> sub, + -> -); it's easy to forget to make one of these changes.