to_string does not output the expected unit when using mm/s or mm/s^2 as a unit
see the example below

auto speedUnit = unit_from_string("mm/s"); cout << "speed unit" << to_string(speedUnit); //outputs mHz*m auto accUnit = unit_from_string("mm/s^2"); cout << "acceleration unit" << to_string(accUnit); //outputs 1/um

the webconverter does the same when pressing to_string, pressing convert will produce the right string representation