Usage: 
{mh, clo, cup} = regxcb(x {,h {,alpha {,K} {,v}}})

Input: 
 x  n x 2, the data. In the first column the
independent and in the second column the
dependent variable are contained.

 h  optional scalar, bandwidth. If not given, 20% of the
range of x[,1] is used as default.

 alpha  optional scalar representing the confidence level;
default alpha = 0.05.

 K  optional string, kernel function on [1,1] with
K(1) = K(1) = 0; default K = "qua".

 v  optional m x 1 vector, values of the independent variable
in which to compute the regression and the confidence
bands. If not given, the (sorted) x is used.

Output: 
 mh  n x 2 or m x 2 matrix; the first column represents
the sorted first column of x or the sorted v and the
second column contains the regression estimate
on the values of the first column. 
 clo  n x 2 or m x 2 matrix; the first column represents the
sorted first column of x or the sorted v and the
second column contains the lower confidence
bounds on the values of the first column. 
 cup  n x 2 or m x 2 matrix, the first column contains the
sorted first column of x or the sorted v and the
second one the upper confidence
bounds on the values of the first column. 