presto-main/src/main/java/io/prestosql/sql/planner/iterative/rule/ReorderJoins.java/JoinEnumerator/toEquiJoinClause
class JoinEnumerator:        private static EquiJoinClause toEquiJoinClause(ComparisonExpression equality, Set<Symbol> leftSymbols)
        {
            Symbol leftSymbol = Symbol.from(equality.getLeft());
            Symbol rightSymbol = Symbol.from(equality.getRight());
            EquiJoinClause equiJoinClause = new EquiJoinClause(leftSymbol, rightSymbol);
            return leftSymbols.contains(leftSymbol) ? equiJoinClause : equiJoinClause.flip();
        }