DECLARE VARIABLE I INTEGER;

OUTPUT = SUBSTRING(INPUT FROM 3); // works

i = 3;
OUTPUT = SUBSTRING(INPUT FROM I); // fails on Token unknown - I

i = 3;
OUTPUT = SUBSTRING(INPUT FROM I); // fails on Token unknown - :

i = 3;
OUTPUT = SUBSTRING(INPUT FROM (I)); // fails on Token unknown - (

I think I've tried all possible combinations :)
Best regards
Hans