This base-catalyzed isomerization involves the following steps: 1. Deprotonation to form tetrahedral intermediate 1; 2. Collapse of the tetrahedral intermediate and bond cleavage to form enolate ion 2; 3. Proton transfer to form enolate ion 3; 4. Intramolecular aldol reaction to form tetrahedral intermediate 4. Write out the reaction on a separate sheet of paper, and then draw the structure of enolate ion 3.