Trace: an inverse coherent unit after a fraction reads ambiguously (20000/413 1/kg)

[christianparpart]

What happens

When a value has no unit with a symbol, a trace writes the coherent unit spelled from its base units: detail::coherent_unit_text in include/formula-cpp/trace_render.hpp. With #12 this covers every dimensioned value without a symbol. A dimension with only negative exponents is spelled 1/X, so in the fraction number style a line can read:

#3 / #1 = 20000/413 1/kg

At a glance, that reads as one fraction divided again: 20000/413 per 1/kg, or 20000/(413·1) per kg. The value is right; the spelling invites a misreading. The same happens for 1/s, 1/m^3, 1/(m s) and so on.

Expected

An inverse unit after a number cannot be read as part of the number. Options:

• negative exponents, kg^-1 (already the caret style the spelling uses for powers);
• a per form, per kg;
• keeping 1/kg only where the number is not a fraction.

The spelling is pinned in test/opaque_tests.cpp (coherent_unit_text) and in trace texts across the tests and guides; they change with it.

Note

This is a spelling choice, not a correctness bug. The issue is here to make that choice deliberately.