DIAG	Create or extract diagonals.
 	If V is a row or column vector with N components,
  	DIAG(V,K) is a square matrix of order N+ABS(K) with the
  	elements of V on the K-th diagonal. K = 0 is the main
  	diagonal, K > 0 is above the main diagonal and K < 0 is
  	below the main diagonal. DIAG(V) simply puts V on the
  	main diagonal. For example,
  	DIAG(-M:M) + DIAG(ONES(2*M,1),1) + DIAG(ONES(2*M,1),-1)
  	produces a tridiagonal matrix of order 2*M+1.
  
  	If X is a matrix, DIAG(X,K) is a column vector formed from
  	the elements of the K-th diagonal of X. DIAG(X) is the main
  	diagonal of X. DIAG(DIAG(X)) is a diagonal matrix.