@BeginModule
@NewPage
@Name      = Maskregion
@Title     = Mask regions
@Section   = Modification
@Class     = Modification
@Arguments = ifile ofile
@Operators = maskregion
@EndModule


@BeginOperator_maskregion
@Title     = Mask regions
@Parameter = regions

@BeginDescription
Masks different regions of a field. The elements inside a region 
are untouched, the elements outside are set to missing value. All 
input fields need to have the same horizontal grid.
The user has to give ASCII formatted files with different regions.
A region is defined by a polygon. Each line of a polygon description file 
contains the longitude and latitude of one point.
Each polygon description file can contain one or more polygons separated
by a line with the character \&.
@EndDescription
@EndOperator


@BeginParameter
@Item = regions
STRING Comma separated list of ASCII formatted files with different regions
@EndParameter


@BeginExample
To mask the region with the longitudes from 120E to 90W and
latitudes from 20N to 20S on all input fields use:
@BeginVerbatim
   cdo maskregion,myregion ifile ofile
@EndVerbatim
For this example the  polygon description file @file{myregion} should contain 
the following four coordinates:
@BeginVerbatim
  120  20
  120 -20
  270 -20
  270  20
@EndVerbatim
@EndExample
