mask {<xray-mask-statement>} end

<xray-mask-statement> :==

  averaging_mode=<logical>
  --{If true: symmetry operations and lattice translations will be encoded
     within the mask region using negative entries. This is required if the 
     mask is to be used for density averaging.}
  to=<real-space-object>
  --{Destination for mask.}
  mode=sigma|vdw|probe|fbox
  --{Method used to determine which points lie within the mask:
     - fbox: box mask: mask<=0 within box extent and 1 outside
     - sigma: molecular mask: use sigma values + probe radius
     - vdw: molecular mask: use vdw radii + probe radius
     - probe: molecular mask: use only probe radius
     Default=vdw}
  xmin=<real> xmax=<real> ymin=<real> ymax=<real> zmin=<real> zmax=<real>
  --{Extent of box in fractional space for mode=fbox. Default=unit cell}
  proberadius=<real>
  --{Probe radius to be added to atomic mask radii. Default=1.4A}
  shrink=<real>
  --{Shrink radius. Default=0}
  selection=<atom-selection>
  --{Atom selection for mask calculation.}
  partition=<integer>
  --{Number of shells to partition points into outside molecular mask.
     Default=1}
  thickness=<real>
  --{Thickness of shells outside molecular mask. Default=1.4A}

  --{The following symbols are declared:
     - $inside: percentage of unitcell volume inside mask
     - $outside: percentage of unitcell volume outside mask
     - $averadius: atomic mask radius averaged over selected atoms
     The resulting map is:
     - 0 within partition 1=(atom+probe-shrink) around atoms
     - 1 within partition 1 and 2
     - n within partition n and n+1
     - the number of shells outside all partitions.}