function x = subdivide2D(x)

% subdivides the rectangles of a list of 
% rectangles (2 x k matrix of intervals)

x1_ = x(1,:); x2_ = x(2,:);
l1 = infsup(inf(x1_),mid(x1_));
r1 = infsup(mid(x1_),sup(x1_));
l2 = infsup(inf(x2_),mid(x2_));
r2 = infsup(mid(x2_),sup(x2_));
x = [l1 l1 r1 r1; l2 r2 l2 r2];

return