Amino Acid Annotation (A3) Format // Implements the Amino Acid Annotation (A3) format using 'S7' classes. The A3 format is a structured schema for annotating amino acid sequences with site, region, post-translational modification (PTM), processing event, and variant information. Provides functions to create, read, and write A3 objects, and to import annotations from 'UniProt', 'AlphaFold', 'PDBe', 'ClinVar', and 'Ensembl'.