DIAG  DIAG Diagonal Matrix Construction/Extraction


Usage
 
 The diag function is used to either construct a 
 diagonal matrix from a vector, or return the diagonal
 elements of a matrix as a vector.  The general syntax
 for its use is
 
   y = diag(x,n)
 
 If x is a matrix, then y returns the n-th 
 diagonal.  If n is omitted, it is assumed to be
 zero.  Conversely, if x is a vector, then y
 is a matrix with x set to the n-th diagonal.
 