1) It assumed left-associativity and was, therefore, wrong for (e.g.)
exponentiation.
2) Arguably, if a user selects `a + |b + c|`, they want to extract `b +
c`, not `a + b + c`. Not being able to do so is surprising (and we may
eventually want to allow it), but so is having the rest of the
least-common subtree extracted.
Fixes#18268