In addition to the AMR labels, a source labelling function is employed, as defined in Section 2.3.
source labels used in CCG/AMR sgraphs take one of two forms: • r, i with r ∈ {root, ∅} and i ∈ N ∪ {∅}, which marks a node as root if r = root, and assigns it an index i if i ∈ N. we disallow ∅, ∅ as a source label.