 1 DIV
 2 ├─ MUL(scalar = 1, powers = (z => 1, 1 + 2x + 3y => 2))
 3 │  ├─ SYM(z)
 4 │  └─ POW
 5 │     ├─ ADD(scalar = 1, coeffs = (y => 3, x => 2))
 6 │     │  ├─ 1
 7 │     │  ├─ MUL(scalar = 2, powers = (x => 1,))
 8 │     │  │  ├─ 2
 9 │     │  │  └─ SYM(x)
10 │     │  └─ MUL(scalar = 3, powers = (y => 1,))
11 │     │     ├─ 3
12 │     │     └─ SYM(y)
13 │     └─ 2
14 └─ ADD(scalar = 0, coeffs = (z => 1, x => 2))
15    ├─ SYM(z)
16    └─ MUL(scalar = 2, powers = (x => 1,))
17       ├─ 2
18       └─ SYM(x)

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