I have very fond memories of Horacio from the time I lived in New York and attended the weekly seminar of R. Parikh’s group, where Horacio was almost always present. We’ve also corresponded a few times in his capacity of editor of JPL and then RSL. But perhaps most important was our South-American complicity, actually. Brazilians and Argentinians may hate each other's guts when it comes to football, but in the outer world we team up for sure...
This is a great shock: Horacio will be dearly missed.
UPDATES: Here is the official obituary from CMU, and here is a post over at Certain Doubts with several initiatives to honor Horacio's life and work, including a CFP for an edited volume in his honor.
ANOTHER UPDATE: Here is Rohit Parikh's eulogy for Horacio.