of :smie-open/close-alist.
(smie-next-sexp): Make it accept a "start token" as argument.
(smie-indent-keyword): Be careful not to misidentify tokens that span
more than one line, as empty lines. Add argument `token'.
Use smie-indent-virtual when indenting relative to an opener.
(smie-rule-separator): Use smie-rule-parent.
(smie-indent-keyword): Consult rules, even for openers at bol.
(smie-indent-comment-close): Try to align closer's content.