I say, and with God's success: The Takbir al-Tahreem is not considered a pillar, but rather a condition of it, and an obligation among its obligations. An obligation does not become a pillar of worship unless it is part of its essence. The Takbir al-Tahreem is the key to prayer, and it is not part of its essence, and it is called 'Tahreem' because it prohibits things outside of prayer. This is the view of the two Sheikhs, Abu Hanifa and Abu Yusuf, while according to Muhammad, the Takbir al-Tahreem is considered a pillar. And God knows best.