# a second copy of the header

One `Ishikawa → scientific method → Six Sigma` loop. The record below is the
commit message as written at the time, before the outcome was known to anyone
reading this file.

## Record — `9cc6040`

```
EXPERIMENT: derive arity from the runtime's own declarations

codegen.el carried builtin_arity(): 344 lines, 300 entries, a hand-maintained
second copy of el_runtime.h.

PREDICTIONS AND RESULTS
  P1 the table duplicates the header                     TRUE   243 shared names
  P2 they have already drifted                           FALSE  ZERO drift. The
                                                                duplicate had been
                                                                maintained correctly.
  P3 codegen can emit call-arity relations               TRUE
  P4 the check becomes a query against the header        TRUE
  P5 codegen drops to roughly baseline                   TRUE   4903 -> 4512,
                                                                149 BELOW the 4661
                                                                it started at

P2 being false is the better result: the table was not WRONG, it was
INCOMPLETE. 110 functions the runtime declares had no entry, so calling them
with the wrong argument count produced no El-level diagnostic at all. Measured:
the old compiler reports 0 arity errors for __http_do_map_to_file(1); the query
reports "takes 5 arguments, called with 1".

Deriving from the header fixes coverage AND makes drift impossible by
construction. 503 signatures, versus 300 entries maintained by hand.

THREE DEFECTS IN MY OWN CHECKER, each found by running it rather than reading it
  1. El names and C names differ -- `println` is `__println`. 60 of 500 decls
     carry the prefix and codegen owns the mapping; the old table carried both
     keys. One rule covers all 60.
  2. Multi-line declarations parsed as zero params, so the checker reported
     "takes 0" for a function taking 5. A diagnostic with the wrong number in it
     is worse than none -- the same shape as the stale caller attribution in the
     previous pass.
  3. Fixing (2) by joining lines dropped 500 signatures to 334, because a
     declaration preceded by a comment no longer started its record. Comments
     are stripped first now.

98/98 native, 5/5 arity_query.sh, fixpoint ok.
```

## Record — `d9e301b`

```
land arity-from-header: the runtime declares its own surface
```