%N region
This selects a subregion of an image. Multiple subregions can be
selected, which are "ored" together. The following subcommands are
recognized (each of which can be abbreviated to uniqueness).

  images(z1,z2)
Select image planes z1 to z2 inclusive. z2 defaults to z1.

  quarter(z1,z2)
Select the inner quarter of the image planes z1 to z2 inclusive.
If both z1 and z2 are missing, then all planes are selected. If
only z2 is omitted, z2 defaults to z1.

  boxes(xmin,ymin,xmax,ymax)(z1,z2)
Select the pixels within a box with corners xmin,ymin,xmax,ymax.
z1 and z2 are the same as in the "image" subcommand. If z1 and z2
are omitted, all planes are selected. If the coordinate system
is a relative one, instead of specifying four values for xmin,
ymin, xmax, and ymax, a single value "m" maybe be specified,
which is exactly equivalent to specifying a box of shape
"-m,-m,m,m".

  polygon(x0,y0,x1,y1,x2,y2,...)(z1,z2)
Select the pixels within the polygon defined by the list of vertices.
z1 and z2 are the same as in the "image" subcommand. If z1 and z2 are
missing, all planes are selected. If only z2 is omitted, it defaults
to z1.

  mask(file)
Select  pixels according to the mask given in the file.

The units of the numbers given in the above commands are, in
general, absolute pixels. But this can be changed (and rechanged)
by using one of the following subcommands.

  abspixel
Coordinates are interpreted as absolute pixel values, the default.
  relpixel
Coordinates are relative to the reference pixel of the map.
  relcenter
Coordinates are relative to the central pixel of the map,
(defined as (naxis1/2+1,naxis2/2+1)).
  arcsec
Coordinates are in arcseconds, relative to the reference pixel.
  kms
Coordinates in the third dimension are in km/s.
