  Let six points $1, ...6$ lie in general position in the real projective plane
and consider the pencil of nodal cubics based at these points, with node at one
of them, say 1. This pencil has five reducible cubics. We call combinatorial
cubic a topological type (cubic, points), and combinatorial pencil the cyclic
sequence of five combinatorial reducible cubics. Up to the action of the
symmetric group $S_5$ on $\{2, ...6\}$, there are seven possible combinatorial
pencils with node at 1. Consider now the set of six pencils obtained, making
the node to be $1, ...6$. Up to the action of S_6 on ${1, ...6}$, there are
four possible lists of six combinatorial pencils. Let seven points $1, ...7$
lie in general position in the plane. Up to the action of $S_7$ on $\{1,
...7\}$, there are fourteen possible lists of seven nodal combinatorial cubics
passing through the seven points, with respective nodes at $1, ...7$.
