_DEFINE lets you define a range of variables according to their first character as a datatype.
Syntax: _DEFINE range or letter[, range2 or letter2[, range3 or letter3[, ...]]] AS datatype
range is in the form firstletter-endingletter (like A-C)
If you want you can just define a single letter as a datatype.
Explanation: Variables that starts with A, B, C or F will now be defined as a unsigned integer, unsigned integers can only take positive values.