Solution (source code)

= Solution

Put $s=\sum_i y_i$. The independent-count <likelihood function> is proportional to $(1-\phi)^n\phi^s$. Its <conjugate prior> is $\boxed{\phi\sim\operatorname{Beta}(a,b),\quad a,b>0}$, giving the <posterior distribution> $\operatorname{Beta}(a+s,b+n)$.