Nickname / diminutive dictionary
nickname_table.RdMaps each canonical given name to its common nicknames and diminutives. Used to
recognise that, e.g., BOB and ROBERT denote the same first name. Some
nicknames map to more than one canonical (e.g. AL -> ALBERT, ALEXANDER);
two names are treated as equivalent when they share any canonical root, so
these ambiguous cases are handled correctly.