Rank Mismatch in Nested Array Reduction

Tacit Reduction of Mixed-Rank Arrays

When utilizing the reduction operator (e.g., +/) on nested arrays in APL, the behavior depends on whether the array is treated as a flat sequence or a nested structure of varying ranks.

A design challenge arises when composing tacit functions that must operate across a specific axis of a nested array without explicitly raveling the structure. If the reduction is applied to a nested array where elements are not of uniform rank, the resulting shape of the output may deviate from the expected axis reduction.

Given the behavior of rank-sensitive functions, how does APL resolve the axis of reduction when the target is a nested array of mixed ranks? Under what conditions will a reduction operator trigger a rank error versus implicitly flattening the nested structure?