  Given a planar digraph $G$ and a positive even integer $k$, an embedding of
$G$ in the plane is k-modal, if every vertex of $G$ is incident to at most $k$
pairs of consecutive edges with opposite orientations, i.e., the incoming and
the outgoing edges at each vertex are grouped by the embedding into at most k
sets of consecutive edges with the same orientation. In this paper, we study
the $k$-Modality problem, which asks for the existence of a $k$-modal embedding
of a planar digraph. This combinatorial problem is at the very core of a
variety of constrained embedding questions for planar digraphs and flat
clustered networks.
