> Does `let s = input_line ic` always increment `pos_in ic` by 
> `String.length s + 1` ?

Only if "ic" is in binary mode.  In text mode, conversions may happen
during reading (e.g. under Windows, removal of the final \r) that
invalidate this claim.

> So, if I define a function
> let undo_input_line ic s = 
> seek_in ic (pos_in ic - String.length s - 1);;
> will it always work as expected, or will this depend on I/O mode (binary vs 
> text) and file format (DOS vs UNIX)?

To "undo" an input, I'd recommend doing "let p = pos_in ic" before the read,
performing the read, and doing "seek_in ic p" to undo the read.
This will be a lot more robust.

- Xavier Leroy
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