presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/PushTopNThroughProject.java/PushTopNThroughProject/symbolMapper
class PushTopNThroughProject:    private Optional<SymbolMapper> symbolMapper(List<Symbol> symbols, Assignments assignments)
    {
        SymbolMapper.Builder mapper = SymbolMapper.builder();
        for (Symbol symbol : symbols) {
            Expression expression = assignments.get(symbol);
            if (!(expression instanceof SymbolReference)) {
                return Optional.empty();
            }
            mapper.put(symbol, Symbol.from(expression));
        }
        return Optional.of(mapper.build());
    }