Syntax:     STR(string,pos,cnt)

Defn: A CI evaluator function that performs general
string extraction. When cnt is positive, cnt
characters are extracted from string. When cnt is
negative, the string that is extracted begins at
pos and ends at the index of the absolute value
of cnt.

Type: String

Example: str('abcde',2,3)
Result: bcd

Example: str('abcdefg',2,-5)
Result: bcde