function [b] = exch_c(a,o)

[nx ny nt]=size(a);
if nt == 1
elseif nt ~= 6
 nt
 error('Must be tiled array (NxNx6)')
end
if nt == 6 & nx ~= ny
 [nx ny]
 error('Tiles must be square (NxNx6)')
end


if nt == 6
b=NaN*ones(nx+2*o,ny+2*o,6);
b(o+1:o+nx,o+1:o+ny,:)=a;				% Interior
b(o+1:o+nx,o+ny+(1:o),1)=a(1:o,ny:-1:1,3)';		% N-odd
b(o+1:o+nx,o+ny+(1:o),3)=a(1:o,ny:-1:1,5)';		% N-odd
b(o+1:o+nx,o+ny+(1:o),5)=a(1:o,ny:-1:1,1)';		% N-odd
b(o+1:o+nx,1:o,[1 3 5])=a(:,ny-o+1:ny,[6 2 4]);		% S-odd
b(o+nx+(1:o),o+1:o+ny,[1 3 5])=a(1:o,:,[2 4 6]);	% E-odd
b(1:o,o+1:o+ny,3)=a(nx:-1:1,ny-o+1:ny,1)';		% W-odd
b(1:o,o+1:o+ny,5)=a(nx:-1:1,ny-o+1:ny,3)';		% W-odd
b(1:o,o+1:o+ny,1)=a(nx:-1:1,ny-o+1:ny,5)';		% W-odd
b(o+1:o+nx,o+ny+(1:o),[2 4 6])=a(:,1:o,[3 5 1]);	% N-even
b(o+1:o+nx,1:o,2)=a(nx-o+1:nx,ny:-1:1,6)';		% S-even
b(o+1:o+nx,1:o,4)=a(nx-o+1:nx,ny:-1:1,2)';		% S-even
b(o+1:o+nx,1:o,6)=a(nx-o+1:nx,ny:-1:1,4)';		% S-even
b(o+nx+(1:o),o+1:o+ny,2)=a(nx:-1:1,1:o,4)';		% E-even
b(o+nx+(1:o),o+1:o+ny,4)=a(nx:-1:1,1:o,6)';		% E-even
b(o+nx+(1:o),o+1:o+ny,6)=a(nx:-1:1,1:o,2)';		% E-even
b(1:o,o+1:o+ny,[2 4 6])=a(nx-o+1:nx,:,[1 3 5]);		% W-even
elseif nt == 1
b=NaN*ones(nx+2*o,ny+2*o);
b(o+1:o+nx,o+1:o+ny)=a;				% 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
b(o+nx+(1:o),:)=b(o+1:o+o,:);			% E
b(1:o,:)=b(o+nx-o+1:o+nx,:);			% W
end