Solution
= Solution
An <index set> is a set $I$ of program indices whose membership is extensional: whenever $f_{u,1}=f_{v,1}$, one has $u\in I$ if and only if $v\in I$. It is nontrivial when it is neither empty nor the set of all indices.
= Solution
An <index set> is a set $I$ of program indices whose membership is extensional: whenever $f_{u,1}=f_{v,1}$, one has $u\in I$ if and only if $v\in I$. It is nontrivial when it is neither empty nor the set of all indices.