One of the tributaries that led to Tongues at the beginning was the idea of a relationship between Prometheus and the Eagle that comes to devour his liver every day. What would they talk about? What kind of relationship would they have? As the first six issues came together, one particular set of conversations between the two of them never quite happened. Not being crucial to the rest of the plot, they kept getting set aside in favor of more pressing narrative concerns. And then at the end of #5 (no spoilers) I realized the story had passed them by. The Tongues Supplement is the answer to this conundrum.