``O.m''

% Overlap operator (acting on 3d data sets)
%
% Usage: out=O(in)
%
% in: input 3d data set
% out: output 3d data set
%
% Uses GLOBAL variable(s) ---
% gbl_R: Lattice vectors

function out=O(in)
  global gbl_R; %# Must declare all globals with such statements to access them

  %# Operator definition (multiplication by volume)
  out= ... %# <=== YOUR CODE HERE
endfunction