SMIE: bare opener forces “hanging” indent

Question

If I use the default SML mode parser, or other SMIE parsers, and have a rule for if-then-else as an expression form, I get reasonable indentation of

   x + if p 
       then
         q
       else
         r

just as I would with parentheses:

   x + (y * 
        z)

However, if I make the opening token (if or () sit on its own, I get indentation of the next token that ignores the position of the opener:

   x + (
     y * 
     z)

I can check for hanging-ness in the rule for (:elem . basic) and indent with respect to the position of the x, but how do I indent with respect to the left-delimiter (if or ()?

I tried

    (`(:elem  . basic) (if (smie-rule-parent-p '("if"))
                           (smie-rule-parent 2)))

but got errors telling me smie--parent wasn't defined.

I also tried to use (smie-rule-parent 2) in a rule for (after . "if") but this didn't help either.

I would like

  x + if 
        p
      then
        q
      else
        r

but don't see how to get it.