Matroids: computing rank of a set

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Hello,

I would like to compute the rank of a given subset of the ground set of a matroid. I've tried

from sage.matroids.advanced import setprint
M = matroids.named_matroids.Vamos();
M.rank_function(['a','b','c'])

but I get the error

AttributeError: 'sage.matroids.circuit_closures_matroid.CircuitClos' object has no attribute 'rank_function'

Is there a way to go around this? Also, since I am very new to Sage: is the matroid package included in Sage by default or do I need to somehow install it? If yes, then how?

Thanks in advance!