If the purchaser of fungible goods commingles such goods with other fungible goods not so purchased but of such similarity that the identity of the constituent goods in the commingled mass cannot be determined and thereafter withdraws for use a part of such commingled goods, the purchaser’s withdrawals shall be deemed to be from the goods not so purchased until a quantity of commingled goods equal to the quantity of goods not so purchased but so commingled has been withdrawn.