+sub __ ($$) { my ($r, $t) = @_; return $r->_ch('gettext')($t); }
+sub _print ($$) { my ($r, @t) = @_; return $r->_ch('print')(join '', @t); }
+
+# pages/param-sets are
+# n normal non-mutating page
+# r retrieval of information for JS, non-mutating
+# m mutating page
+# u update of information by JS, mutating
+# i login
+# o logout
+# O "you have just logged out" page load
+
+# in cook and par,
+# a, aN anything including -
+# t, tN temporary value (in our db, no logged in user yet)
+# y, yN value corresponds to logged-in user
+# n, nN value not in our db
+# x, xN t or y
+# - no value supplied
+# if N differs the case applies only when the two values differ
+# (eg, a1 y2 does not apply when the logged-in value is supplied twice)
+
+# "stale session" means request originates from a page from a login
+# session which has been revoked (eg by logout); "cleared session"
+# means request originates from a browser which has a different (or
+# no) cookie.
+
+ # Case analysis, cookie mode, app promises re mutate:
+ # cook parm meth form
+ #
+ # any - POST nrmuoi bug or attack, fail
+ # any - GET rmuoi bug or attack, fail
+ # any any GET muoi bug or attack, fail
+ # any t any nrmu bug or attack, fail
+ #
+ # - - GET O "just logged out" page
+ # (any other) O bug or attack, fail
+ #
+ # a1 a2 POST o logout
+ # if a1 is valid, revoke it
+ # if a2 is valid, revoke it
+ # delete cookie
+ # redirect to "just logged out" page
+ # (which contains link to login form)
+ #
+ # - t POST i complain about cookies being disabled
+ # (with link to login form)
+ #
+ # any n POST i complain about stale login form
+ # show new login form
+ #
+ # x1 t2 POST i login (or switch user)
+ # if bad
+ # show new login form
+ # if good
+ # revoke x1 if it was valid and !=t2
+ # upgrade t2 to y2 in our db (setting username)
+ # set cookie to t2
+ # redirect to GET of remaining params
+ #
+ # t1 a2 ANY nrmu treat as - a2 ANY
+ #
+ # y - GET n cross-site link
+ # show data
+ #
+ # y y GET nr fine, show page or send data
+ # y y POST nrmu mutation is OK, do operation
+ #
+ # y1 y2 GET nr request from stale page
+ # do not revoke y2 as not RESTful
+ # treat as y1 n GET
+ #
+ # y1 y2 POST nrmu request from stale page
+ # revoke y2
+ # treat as y1 n POST
+ #
+ # y n GET n intra-site link from stale page,
+ # treat as cross-site link, show data
+ #
+ # y n POST n m intra-site form submission from stale page
+ # show "session interrupted"
+ # with link to main data page
+ #
+ # y n GET r intra-site request from stale page
+ # fail
+ #
+ # y n POST r u intra-site request from stale page
+ # fail
+ #
+ # -/n y2 GET nr intra-site link from cleared session
+ # do not revoke y2 as not RESTful
+ # treat as -/n n GET
+ #
+ # -/n y2 POST nrmu request from cleared session
+ # revoke y2
+ # treat as -/n n POST
+ #
+ # -/n n GET n cross-site link but user not logged in
+ # show login form with redirect to orig params
+ #
+ # -/n n GET rmu user not logged in
+ # fail
+ #
+ # -/n n POST n m user not logged in
+ # show login form
+ #
+ # -/n n POST r u user not logged in
+ # fail
+
+sub _check_divert_core ($) {
+fixme needs wrapping with something to make and commit a transaction