This function is provided by the package Matroids.
A matroid N is a coextension of the matroid M if the dual of M is an extension of the dual of N. The free coextension of M is the dual of the free extension of the dual of M. The free coextension of a uniform matroid U_{r,n} is U_{r+1,n+1}.



The free coextension N is a matroid whose ground set is the ground set of M plus one additional element e. Flats of the free coextension come in one of two types: They are either independent sets of M or sets containing e that become flats of M after removing e.





The object coextension is a method function.