#1 Min/Max entry for list

[Cinly Ooi]

For list-based input, at present, one can choose
whether an empty list is allowed. This facility can be
generalized to allow a min/max value to be set. Example
where this can be useful, using BrainFX as an example:

No value:
Optional input. Sometimes useful when list can be empty.

Minimum of two value:
When doing a paired 2x1 ANOVA, having one item only is
not correct and can cause programs to fail unexpectedly.