give example of "bad" solve_mod use

[kcrisman]

While solve_mod explicitly states

Each equation must involve only polynomials in 1 or many variables.

it might be helpful to have a test for that particular error with, say, a discrete log thing like solve_mod(5^x==2,23) or the like. People often read the documentation too quickly, but if they then come back and find the error already in the doc, it can be useful, and point them to a better command, such as discrete_log(2,mod(5,23)), in that case, which will also detect when there is not a solution.

[mkoeppe]

Moving to 9.4, as 9.3 has been released.