-- >>> let f1 = fwd "a@example.com" "a@example.com"
-- >>> let f2 = fwd "a@example.com" "a1@example.net"
-- >>> let f3 = fwd "a@example.com" "a2@example.org"
-- >>> map pretty_print (dropby_goto_domains ds [f1,f2,f3])
-- ["a@example.com -> a2@example.org"]
--
-- >>> let f1 = fwd "a@example.com" "a@example.com"
-- >>> let f2 = fwd "a@example.com" "a1@example.net"
-- >>> let f3 = fwd "a@example.com" "a2@example.org"
-- >>> map pretty_print (dropby_goto_domains ds [f1,f2,f3])
-- ["a@example.com -> a2@example.org"]
--