title: Multiple leaflets in a grid
body: <p>I have 4 leaflet objects: A, B, C, D. I would like to plot them in a 2 by 2 grid, but I have been having a difficult time trying to do this. </p>
  
  <p>My initial thought was to use ggplot and facet_grid, but ggplot does not know how to deal with objects of class  leaflet.</p>
  
  <p>I would appreciate the help!</p>