function X = chop(Xi,n,u)
if nargin<3,
   u=1; 
end
X = abs(Xi) +(Xi==0);
[nx,mx] = size(X);
exponent = u.*((10*ones(nx,mx)).^(floor(log10(X))-n+1));
X = round(X./exponent).*exponent;
X = sign(Xi).*X.*(Xi~=0);