Standing in front of the unfinished Alchemist’s Shack with artist and Mildred’s Lane codirector J. Morgan Puett, I realized that some things can take forever. An art installation-cum-habitation, the shack has been a work in progress for Puett, artist and codirector Mark Dion, artist Robert Williams, and the countless participants who have visited Mildred’s Lane for nearly twenty years. Over …