Certain procedures, such as list-ref, string-ref and vector-ref, work with an indexed character or item within an object. It is important to know that all indices in Mosquito Lisp start from 0.
So:
The indices for the string "MOSREF" would be
M=0, O=1, S=2, R=3, E=4, and F=5.
If you wanted the "R" from "MOSREF", you would use
(string-ref "MOSREF" 3)
while to retrieve the "M", you would use 0 as the integer in that expression.
Let's look at another procedure:
(list-ref '(a b c) 1) returns b
In all cases, the index reference must be >=0 AND a valid index for the object. So,
(list-ref '(a b c) 7) returns an error.