A spanning forest of the part of the graph reachable
from the listed vertices, obtained from a depth-first search of the
graph starting at each of the listed vertices in order.
dfs next found initial performs a depth-first search over a
set of states, starting with initial and generating
neighboring states with next. It returns a depth-first path
to a state for which found returns True. Returns
Nothing if no path is possible.
The URL where the feed file can be fetched. Google Merchant Center
will support automatic scheduled uploads using the HTTP, HTTPS, FTP,
or SFTP protocols, so the value will need to be a valid link using one
of those four protocols.