Syntax:     REPL(str,oldstr, newstr[,cnt] [,start])

Defn: A CI evaluator function that, in a given string (str),
replaces cnt occurrences of oldstr with newstr,
beginning at start. If cnt is positive, replacement
begins at the left end of str; if negative, replacement
begins at the right end of str. Default start is 1;
default cnt is zero (meaning all occurrences).

Type: String

Example: repl('aaabcaab','aa','X')
Result: 'XabcXb'

Example: repl('aaabcaab','ab','',-1)
Result: 'aaabca'