However, the Clementine Vulgate version opens itself to a very beautiful allegorical reading: "beatus qui tenebit et adlidet parvulos tuos ad petram."
We are encouraged to dash the infants of our enemies "ad petram."
Now couple this with the Vulgate version of Matthew 16:18
"et ego dico tibi quia tu es Petrus et super hanc petram aedificabo ecclesiam meam et portae inferi non praevalebunt adversum eam."
To read it allegorically, we should be asking that the infants of our enemies be dashed against Peter and the foundation of the Catholic Church! For example, see the photo at that top of this post--that's Pope Benedict's hand baptizing an infant.
It's edifying (nerdy Latin Vulgate pun intended) to pray Psalm 136 with Mt 16:18 in mind, and then intend that the children of our enemies (secularists, terrorists, haters of the Church, those who have hurt us) be thrown against Peter and the Church...that they be baptized, saved, and remain within the barque of Peter...
The Psalms are so rich. It's too bad that Psalm 136:9 has been removed from the Liturgy of the Hours. A true pity.