### diag

 description:D = diag(A,k) produces diagonal matrices or extracts diagonals of a matrix A.diag(V,K) when V is a vector with n components 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) is the same as diag(V,0) and puts V on the main diagonal.diag(X,k) when X is a matrix is a column vector formed from the elements of the k-th diagonal of X.diag(X) is the main diagonal of X.arguments:A is a vector or matrix of NCpolysk is an integeroutput:vector or matrix of NCpolyspossible usage:diag(A), diag(A,k)example:>> diag([x x^2 x*y-y*x])ans =  x     0         0       0   x^2         0       0     0   x*y-y*x>> diag([x x^2 x*y-y*x],1)ans = 0   x     0         0      0   0   x^2         0      0   0     0   x*y-y*x      0   0     0         0>> A=[x x^2 x*y-y*x; y x*y 2*x-y]A =  x   x^2   x*y-y*x     y   x*y     2*x-y>> diag(A)ans =   x      x*y>> diag(A,1)ans =   x^2      2*x-y