function [b] = exch_llc(a,o)

[nnx nny]=size(a);

if (nnx/4~=round(nnx/4))
  error('x-dimension needs to be divisible by 4')
end
n4 = nnx/4;
% pull array appart
ca  = a(:,end-n4+1:end);
lla = a(:,1:end-n4);
% do the tricky bit first because we need the ll part for that
[nx,ny]=size(ca);
lln = lla(:,end-o+1:end);
cb = NaN*ones(nx+2*o,ny+o);
% interior
cb(o+1:nx+o,1:ny)=ca;
%
cb(1:o,:) = flipud(lln(n4*3-o+1:n4*4,:))'; % right
cb(end-o+1:end,:) = fliplr(lln(1:n4+o,:))'; % left
% top
cb(o+1:o+n4,end-o+1:end) = flipud(fliplr(lln(2*n4+1:3*n4,:)));
cb(o+n4+1:o+2*n4,end-o+1:end) = fliplr(flipud(lln(n4*3+1:n4*4,:)));
cb(o+n4*2+1:o+3*n4,end-o+1:end) = flipud(lln(1:n4,:));
cb(o+n4*3+1:o+4*n4,end-o+1:end) = flipud(lln(2*n4+1:3*n4,:));

% now exchange regular part:
[nx,ny]=size(lla);
llb=NaN*ones(nx+2*o,ny+o);
llb(o+1:o+nx,o+1:o+ny)=lla;			% Interior
%b(o+1:o+nx,o+ny+(1:o))=a(:,ny:-1:ny-o+1);	% N
%b(o+1:o+nx,1:o)=a(:,o:-1:1);			% S
llb(o+nx+(1:o),:)=llb(o+1:o+o,:);		% E
llb(1:o,:)=llb(o+nx-o+1:o+nx,:);		% W

% put both parts together

b = [llb cb];