HIRS includes are split into several files.  If you want a complete and
ready run, include either "hirs_fast.arts" (fast setup) or
"hirs_reference.arts" (reference, slow setup).  Read the respective files
for information on what to set before including them.  These include files
perform the following tasks:

- set some general settings
- set the spectroscopic parameters
- reads hitran
- defines the sensor (reference or fast)

In practice, one might, for example, want to use HIRS without reading the
HITRAN, for example to calculate a lookup table for later use.  In that
case, you can include "lower-level" files, such as:

INCLUDE "hirs/hirs_general.arts"
INCLUDE "hirs/hirs_spectroscopy.arts"
INCLUDE "hirs/hirs_sensor_common.arts"
INCLUDE "hirs/hirs_sensor_fast.arts"

In fact, this is exactly what hirs_reference.arts and hirs_fast.arts do,
except that they also include "hirs/hirs_hitran.arts" (as number three).