1 ADD(scalar = 0, coeffs = (z => 1, x => 2))
2 ├─ MUL(scalar = 2, powers = (x => 1,))
3 │  ├─ 2
4 │  └─ SYM(x)
5 └─ SYM(z)

Hint: call SymbolicUtils.pluck(expr, line_number) to get the subexpression starting at line_number