how should a program ask for no SI prefix?

[walter b]

Hmmh, still not sure I understand what you want. Let me try it this way:

If you present something like a dropdown menu letting the user choose a prefix then I'd suggest P/T/G/M/k/ /m/µ/n/p/f if this would cover the needs (watch the lower case k!). If a blank character is forbidden for any reason whatsoever I'd suggest printing "none" as an alternative.

HTH a bit

d:-?