chore(program): a frame stays a set of dimension names, and only the sum over a join reads the axes it opens

[FBumann]

Prompt: Please review the PR stack 665. Can we simplify it with a stacked PR? […] Do it as a stacked draft pr so i can judge the diff

Note

The following content was generated by AI.

On top of #665, frames stay sets of dimension names. dims_of reads a sum over a join as one rule, so no frame holds a join's axis. Axis and Column stay in Sum.over and JoinColumns.axes. #665 shrinks from +183/−107 to +106/−61.

What this changes

• dims_of returns frozenset[str] again. For a Sum over a Join that opens axes, it returns the join's frame without them. A Join that opens axes and has no sum over it is an assert.
• frame_of, _names and _dimensions are removed. The Axis(d) in frame tests in _where_resolver.py, lowering.py and piecewise.py, and the str(axis) in composition.py, are removed too. These four files are the same as on feat(language)!: a call names the columns of a relation as relation[column], and a sum through a relation is a sum over the axes its join opens #664 again.
• Unchanged from feat(program): the axis a join opens names the relation column it stands for #665: Axis, Column, Sum.over: tuple[Axis, ...], JoinColumns.axes, the separability and typesetter changes, and the docs.

Why

The axes a join opens never leave the sum that closes them. The resolver is the only code that builds a Sum over a Join, and it always sums over exactly join.axes (_expression_resolver.py:305). The typesetter (walk.py:421) and separability already read the pair as one unit. So the dimension check does not need axes in its frames. Three of the five guards in #665's table exist only because a string tested against a set of axes is always false. With string frames, that kind of error cannot happen.

Size, guards, coverage moved, gates, not done

Size, src/ and tests/, against #664's head

	files	lines
#665 as is	13	+183 / −107
#665 with this PR	9	+106 / −61

dimensions.py goes from about 98 changed lines to 30.

Guards

This PR adds two asserts. Each one states something that the resolver makes true:

• _sum_dims: a sum over a join closes exactly join.columns.axes.
• dims_of: a join that opens axes is read only by the sum that closes them.

No test fails when either assert is deleted, because no input reaches its false arm. I did not add a probe that builds such a Sum by hand.

#665's last three guards (Axis(d) in carried, in found, in dims) no longer exist, because those lines test a string against a set of strings again.

Coverage that moved

• test_a_join_opens_an_axis_for_the_column_it_drops_and_the_sum_over_it_closes_it no longer checks the frame of the bare join, because that frame no longer exists. It still checks node.over == (Axis('snapshot', Column('rep_of', 'snapshot')),) and the frame of the sum.
• The other tests/test_dimensions.py assertions compare to string sets again, as on feat(language)!: a call names the columns of a relation as relation[column], and a sum through a relation is a sum over the axes its join opens #664.

Gates

Pixi is not reachable here. The gates ran from a uv venv on Python 3.12, with the pinned ruff and pyrefly.

gate	result
pytest -q -n auto	2557 passed, 53 skipped
ruff check, ruff format --check .	clean
pyrefly check on src/	4 errors, all about missing yaml stubs in the venv. I did not compare this to #665's head.
docs-build, compile-tex, the git hooks	not run

Not done

• No CHANGELOG.md line. dims_of is not public API, so this is chore. If this PR is folded into feat(program): the axis a join opens names the relation column it stands for #665, feat(program): the axis a join opens names the relation column it stands for #665's title and changelog line should drop "a frame holds axes".
• feat(program): the axis a join opens names the relation column it stands for #665's body is not changed.
• feat(language)!: a call names the columns of a relation as relation[column], and a sum through a relation is a sum over the axes its join opens #664 cleanups are not included. A review found about 40 lines, for example JoinColumns.kept_dims, which has no reader. That is separate work.

🤖 Generated with Claude Code

https://claude.ai/code/session_01CPwrjirBZhVS4CcndN6f4H

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Documentation build overview

📚 mathspec | 🛠️ Build #34855794 | 📁 Comparing 495f3da against latest (4581220)

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42 files changed · + 1 added · ± 41 modified

+ Added

• about/relations-as-linear-maps/index.html

± Modified

• changelog/index.html
• contributing/index.html
• several-files/index.html
• about/limits/index.html
• about/what-counts-as-language/index.html
• examples/operators/index.html
• examples/pypsa_linearized_uc/index.html
• howto/compose/index.html
• howto/curve-by-hand/index.html
• howto/declare-a-column/index.html
• and 31 more...