  An SPQR-tree is a data structure that efficiently represents all planar
embeddings of a biconnected planar graph. It is a key tool in a number of
constrained planarity testing algorithms, which seek a planar embedding of a
graph subject to some given set of constraints.
  We develop an SPQR-tree-like data structure that represents all level-planar
embeddings of a biconnected level graph with a single source, called the
LP-tree, and give a simple algorithm to compute it in linear time. Moreover, we
show that LP-trees can be used to adapt three constrained planarity algorithms
to the level-planar case by using them as a drop-in replacement for SPQR-trees.
