sage: f = function('f', x)
sage: divided_difference([x], [f(x=x)])
f(x)
sage: f = function('f', x)
sage: divided_difference([x], [f(x=x)])
f(x)
sage: divided_difference([x1,x2], [f(x=x1),f(x=x2)])
f(x1)/(x1 - x2) - f(x2)/(x1 - x2)
sage: divided_difference([x1,x2], [f(x=x1),f(x=x2)])
f(x1)/(x1 - x2) - f(x2)/(x1 - x2)