%N line
The ``line'' parameter determines the channels that are to be processed
from a uv data-set. The parameter value consists of a string
followed by up to four numbers. Defaults are used for any missing
trailing portion of the parameter value.

A uv data-set can have correlations from either (or both) a spectral
or a wideband (continuum) correlator. Both the spectral and wideband
correlators produce multiple correlations (channels). The string
part of the line parameter is used to select the spectral or wideband
channels. It may be one of:
  "channel"   Spectral channels.
  "wide"      Wideband (continuum) channels.
  "velocity"  Spectral channels, resampled at equal increments in
              velocity (using the radio definition). The resampling
              involves a weighted average of the spectral channels.
              This is useful if the source was not Doppler tracked
              correctly.
  "felocity"  Similar to "velocity", except that the parameters are
              given using the optical velocity definition.
Generally the default is "channel" if the data-set has spectral
channel data, and "wide" otherwise.

The four numbers that accompany the string give:

  nchan, start, width, step

These four numbers specify which channels to select from the input
dataset and how to combine them to produce the output channels that
the Miriad task will work on.

nchan   is the number of output channels produced. Generally it
        defaults to the number of input channels.

start   is the first channel from input dataset that is to be used.
        It defaults to 1 (i.e. first channel).

width   gives the number of input channels to average together to
        produce a single output channel. It defaults to 1.

step    gives the increment between selected input channels. It
        defaults to the value of "width".

For "velocity" linetype, the start, width and step parameters are
given in km/s. The output channels are centered on velocities:
start, start+step, start+2*step, etc.

The `line' parameter interacts with the "select=window" selection
for "channel" and "velocity"/"felocity" linetypes. See the help on
select for more information.

For example:

  line=channel,10

selects 10 output channels, being input spectral channels 1 to 10.
Similarly

  line=channel,10,8,1,2

selects 10 output channels, starting at input channel 8, and skipping
every second input channel, whereas

  line=channel,10,8,2,2

selects 10 output channels, again starting at input channel 8, but
each of the output channels consists of the average of two of the
input channels. Finally

  line=velocity,10,-10,1,1

resamples the spectral data in velocity, to give 10 channels of width
1 km/s. The channels are centered at -10.0,-9.0,-8.0, etc, km/s.
