Three-argument filtered map macro raises ValueError during evaluation

Start by running the provided repro.py against the map macro evaluator and trace the three-argument filtered map path described in the issue. Add coverage for both expressions, then confirm the filtered form returns 3 instead of raising ValueError while the existing filter(...).map(...) form still passes.

Summary

The standard filtered map(x, predicate, transform) form compiles but raises ValueError: too many values to unpack (expected 2) during evaluation. The equivalent filter(...).map(...) expression completes normally.

Reproduction

Save the following as repro.py.

from celpy import Environment

for expression in (
    "[1,2,3].map(x, x > 1, x+1)[0]",
    "[1,2,3].filter(x, x > 1).map(x, x+1)[0]",
):
    env = Environment()
    try:
        result = env.program(env.compile(expression)).evaluate({})
        print(expression, "=>", result)
    except Exception as exc:
        print(expression, "=>", type(exc).__name__, str(exc))

Run:

python -m pip install cel-python==0.5.0
python repro.py

Actual result

[1,2,3].map(x, x > 1, x+1)[0] => ValueError too many values to unpack (expected 2)
[1,2,3].filter(x, x > 1).map(x, x+1)[0] => 3

Expected behavior and why

Both expressions should return 3: filter [1,2,3] to [2,3], add one to each element, and select the first result.

CEL defines e.map(x, p, t) as the map transformation with predicate filtering before transformation:
https://github.com/cel-expr/cel-spec/blob/ba58ae5007845f3a1279b488cdeb79645ce958bb/doc/langdef.md#macros

Code-level observation and impact

The map evaluator unpacks two macro arguments where this form supplies three. This is an internal Python arity error rather than the normal evaluation of a supported CEL expression. If this macro overload is intentionally outside the supported profile, please clarify that scope; accepting the syntax and then raising an unpacking error is still an unhelpful failure mode for callers.

Environment

• Package: cel-python 0.5.0
• Runtime: CPython 3.13.3
• OS: macOS 26.6.2, Apple Silicon / arm64
• Reproduced: 2026-09-11

cel-python-reproduction.zip