> I guess I was actually on FB2.5. After switching back to FB1.5, I think I'm out of luck because the SUBSTRING internal function
> requires integer literals. So this statement isn't possible:
>
> substring( input_string from idx-1 for 1 )

To get n-th character from string, just remove first n-1 characters by (n-1)x calling
tmp_string = substring(tmp_string from 2);
and then get first one by
substring(tmp_string from 1 for 1 )

Ivan