presto-main/src/main/java/io/prestosql/sql/planner/optimizations/UnaliasSymbolReferences.java/Rewriter/canonicalize
class Rewriter:        private Set<Symbol> canonicalize(Set<Symbol> symbols)
        {
            return symbols.stream()
                    .map(this::canonicalize)
                    .collect(toImmutableSet());
        }