PROB
*This needs to specify if Data must be ordered or not (unordered?).
*It would also be better to be clear about whether Data must be
sequential (is 1, 2, 3, 5 (missing 4) OK?) and/or integer (is 1, 2.5, 3,
4 OK?).
*It would also be better to be clear about whether Start and End need to
exist within Data (no?) and also whether they can be higher/lower than
the max/min values in Data (yes?)

• It would also be better to be clear about whether Start can be higher
  than End. Although they're defined as lower/upper bounds, is 'Start>End'
  an error, or valid with a probability of 0 returned?
• This needs to specify how the result is calculated if End is omitted
  and Start is not present in Data (Calc returns 0).
• This needs to specify the type of error returned - eg #N/A if Data,
  Probability are different lengths. This is important because #N/A is a
  specific (testable) type of error.