           key = "#{v2},#{v1}"
         if key of edgeMap
           edge = edgeMap[key]
+          # Second instance of edge means not on boundary
+          mesh.edges_assignment[edge] = null
         else
           edge = edgeMap[key] = mesh.edges_vertices.length
           if v1 <= v2

convert.faces_vertices_to_edges = (mesh) ->
  ###
  Given a FOLD object with just `faces_vertices`, automatically fills in
  `edges_vertices`, `edges_faces`, `faces_edges`, and `edges_assignment`.
  ###
  mesh.edges_vertices = []
  mesh.edges_faces = []

          else
            mesh.edges_vertices.push [v2, v1]
          mesh.edges_faces.push [null, null]
          # First instance of edge might be on boundary
          mesh.edges_assignment.push 'B'
        if v1 <= v2
          mesh.edges_faces[edge][0] = face
