edited
Given that the words "uncertain" and "uncertainty" are fairly ubiquitous, it's hard to Google "uncertainty arithmetic" and get anything immediately helpful. Thus, can anyone suggest a good library of routines, in almost any programming/scripting language, that implements handling of uncertain values, as per this description:
Use uncertainty arithmetic to record values that are approximations, for which there is a measured tolerance. This is when we are unsure about a value, but know the upper and lower bounds it can have, expressed as a ±value.
I believe "Interval Arithmetic" is the more common name for what you're looking for. boost::interval would be my first choice for a supporting library.