presto-main/src/main/java/io/prestosql/sql/planner/QueryPlanner.java/QueryPlanner/symbolsForExpressions
class QueryPlanner:    private static Map<Expression, Symbol> symbolsForExpressions(PlanBuilder builder, Iterable<? extends Expression> expressions)
    {
        return stream(expressions)
                .distinct()
                .collect(toImmutableMap(expression -> expression, builder::translate));
    }