presto-main/src/main/java/io/prestosql/sql/planner/optimizations/UnaliasSymbolReferences.java/UnaliasSymbolReferences/map
class UnaliasSymbolReferences:        private void map(Symbol symbol, Symbol canonical)
        {
            Preconditions.checkArgument(!symbol.equals(canonical), "Can't map symbol to itself: %s", symbol);
            mapping.put(symbol, canonical);
        }