How to get all diagnoal elements in a matrix?

Question

I'd like to write a function that determines all the diagonal elements in a matrix are same.

For instance:

?-diagonal([[1,2,3,4],
            [2,1,5,6],
            [6,2,1,9],
            [8,7,5,1]]).
  true.

Any help is appreciated.

Answer 1

Question solved...

diagonal([],_,_).
   diagonal([Head|Tail],Index,Value),
   nth0(Index,Head,Value),
   IndexNext is Index+1,
   diagonal(tail,IndexNext,Value).

Test:

?- diagonal([[1,2,3],
             [5,1,6],
             [9,3,1]],0,X).
   X=1.