& is also used for set intersection in Python. I think + for string concatenation isn't too bad, really. It fits in with the fact that length(s + t) = length(s) + length(t), the same way we write A × B for Cartesian product (since |A × B| = |A| × |B|, even though this operation is neither commutative nor associative) or B^A for a function space (since |B^A| = |B|^|A|).