presto-main/src/main/java/io/prestosql/sql/planner/plan/Assignments.java/Assignments/toAssignments
class Assignments:    private Collector<Entry<Symbol, Expression>, Builder, Assignments> toAssignments()
    {
        return Collector.of(
                Assignments::builder,
                (builder, entry) -> builder.put(entry.getKey(), entry.getValue()),
                (left, right) -> {
                    left.putAll(right.build());
                    return left;
                },
                Assignments.Builder::build);
    }