**Challenge Description**:
Given output from a program written in Opal (a functional language), decode it to recover the flag.

**Analysis**:
The challenge provides:
- `challenge_final.impl`: Source code in Opal language
- `OUTPUT.txt`: Shows Base64-encoded output `YnpYZVeGc45lc2VUZ05h`

The Opal program performs:
1. `f1`: Substitution cipher (char → char mapping)
2. `f3`: Applies `f1` to each character
3. `f4`: Converts string to sequence of nats
4. `f8`: Accumulator-based transform with `f6` (even/odd multiplier)
5. `f13`: Base64 encoding

**Solution**:
1. Decode Base64 to get intermediate bytes
2. Brute-force the accumulator sum `S` (0-255) to invert `f8`
3. Invert the substitution cipher `f1`

**Solver**: [solve_opalist.py](file:///home/mritunjya/ctf/2026/nullcon/rev/opalist/solve_opalist.py)

**Flag**: `ENO{R3v_0p4L_4_FuN!}`