A third equation-editor session (corpus/math-edit/native-editor-3) adds a
matrix, an equation array, upper and lower limit objects, padded and boxed
formulas, a nested fraction and a sum with only an upper limit to the
MathML oracle. The model carries a matrix or array's column count
(MathObject::columns, text-run data 0x0c003451, written back by the
equation pass), and the mapping reproduces all ten exports: mtable cells
row by row, equation-array rows with alignment groups and a mark for each
ampersand, mpadded and menclose boxes, and n-ary operators that leave an
empty limit out (mover or msup, munder or msub). Every object kind the
editor produced now has a native check.

Assisted-by: claude-fable-5.1