I was able to isolate the error at y(0,0) to the "or(a,b)" function in "y(a,b)". You are getting this error because you need to use logical operators (and, or, ~, |, &, ...) with symbolic expressions and variables. Always make sure that you use logical expressions, such as:
"x(0,0) does not cause an error because "xor" in the Symbolic engine works not only on logical values, but also accepts numbers. This does not match other the other functions, so you should try to always use logical expressions.