Is someone who missed the opening "takbir" (Allahu Akbar) considered to have missed it because he was performing a supererogatory prayer in the mosque when the congregational prayer began?

If you enter the mosque and begin a supererogatory prayer (nafilah), and then the obligatory prayer (fard) is called, if it is possible to complete the nafilah before the takbirat al-ihram (definition) (opening takbir) of the obligatory prayer, then do so. Otherwise, interrupt the nafilah and catch the takbirat al-ihram with the Imam, because the preponderant opinion is that whoever has the obligatory prayer established upon them while they are performing a supererogatory prayer should interrupt the supererogatory prayer.