An outline of Cross. For illustration purposes we assume here σ = 1. The routines SHADOW, SKELETON BRANCH and SATURATION are depicted. Due to space limitations we only represent a few select motifs and for the same reason only one of the motifs displayed in the first row is extended by one arc (drawn in blue). Furthermore note that only motifs with crossings give rise to nontrivial skeleton-trees, all other motifs are considered directly as input for SATURATION.