For questions related to the XOR problem, i.e. the problem of computing/approximating the XOR logical function, i.e. a function $f: \{0, 1\}^2 \rightarrow \{0, 1\}$ that returns 1 only when the two bits are different and 0 otherwise, i.e. $f(a, b) = 1 \iff a \neq b$ and $a, b \in \{0, 1\}$.