error in Posets examples (order ideal function)

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Hi!

I am very new here! :) I am trying to use sage to work with posets and lower sets (order ideal I think it is the same). My sage version is 5.4.1. I want all the lower sets of a Poset P, for example. I found in an example this:

 P = Poset((divisors(12), attrcall("divides")), facade=True)
 A = P.directed_subsets('up')
 sorted(list(A))
 [[], [1, 2, 4, 3, 6, 12], [2, 4, 3, 6, 12], [2, 4, 6, 12], [3, 6, 12], [4, 3, 6, 12], [4, 6, 12], [4, 12], [6, 12], [12]]

That looks like what I want! BUT, when I run the example in my sage it gives me an error!:

sage: P = Poset((divisors(12), attrcall("divides")), facade=True)
sage: A = P.directed_subsets('up')
AttributeError                            Traceback (most recent call last)

/Users/edwardo/<ipython console> in <module>()

/Applications/sage/local/lib/python2.7/site-packages/sage/structure/parent.so in sage.structure.parent.Parent.__getattr__ (sage/structure/parent.c:5969)()

/Applications/sage/local/lib/python2.7/site-packages/sage/structure/misc.so in sage.structure.misc.getattr_from_other_class (sage/structure/misc.c:1427)()

AttributeError: 'FinitePoset_with_category' object has no attribute 'directed_subsets'

It looks like 'directedsubsets' does not work... Why it does not work?? How I can fix this? I have to download a library or something?? Here is the link to the example:

  http://www.sagemath.org/doc/reference/categories/sage/categories/finite_posets.html

Help! Thanks!