You can understand that x can take any value, so it can be represented by c and y, but according to the discrete book, the renaming rule is used. Want to further understand, is the problem of free variables and constrained variables. The reason why you sometimes write Y and sometimes C is because people can understand whether it is free variables or constraint variable in operation, just like when you seek the toe-in paradigm, you use X and Y in front, so you use C (generally, use X, Y first, then C). In fact, you can use c or y, and the most important thing is to make people understand that this is a free argument and a binding argument. I hope it helps you.