#include <IO.h> int io_mod_extension( GapIO *io, int N, int shorten_by);
io_mod_extension modifies the position of the 3' cutoff data for
reading number N. The 3' cutoff position is defined to be the base
number, counting from 1, of the first base within the cutoff data.
shorten_by is subtracted from either the end or start field in the GReadings structure, depending on whether the reading is complemented. It is legal to specify a negative amount to increase the used portion of the reading.
NOTE that this implementation does not set the sequence_length field or
io_length(io,N) data for this reading.