matrixDiagonal

def matrixDiagonal(matrix,x,y):
#@param:
#   matrix: matrix;
#   k: int;
    assert matCheck(matrix), "Matrix is not square."
    assert x<len(matrix) and y<len(matrix) and x>=0 and y>=0
    i = 0
    while i<len(matrix)-x and i<len(matrix)-y:
        matrix[x+i][y+i] = 0
        i += 1

def matCheck(matrix):
#@param:
#   matrix: matrix;
#   return: bool;
    lenght = len(matrix)
    for i in range(0,lenght):
        if len(matrix[i]) != lenght:
            return False
    return True