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

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