%N stokes
This keyword specifies the Stokes/polarisations parameters that
are to be processed. Tasks which use this keyword can usually convert
raw polarisations to Stokes parameters (but not the other way).
Generally several values can be given, separated by commas.
Possible values are:
  For raw linear polarisations:             xx,yy,xy,yx
  For raw circular polarisations:           rr,ll,rl,lr
  For Stokes parameters:                    i,q,u,v
  For Stokes I of an unpolarised source:    ii
The "ii" form gives Stokes I, using the assumption that the
source is unpolarised.

There is an alternate way to specifiy the desired
Stokes/polarisation data, using `select' (see the help on select).
The `select' mechanism is rather different, and may be more or
less appropriate to the job in hand.

Examples:

To process the 4 Stokes parameters:
  stokes=i,q,u,v

To process just the linear parallel hand polarisations:
  stokes=xx,yy

To process both the linear parallel hands, and Stokes I:
  stokes=xx,yy,i

NOTE: Some tasks restrict or prohibit the concurrent use of stokes
and select=pol.
