How does SMIE mode determine what the parent of a token is?

Question

I'm writing indentation rules for a custom mode using SMIE.

I want to use smie-rule-parent-p to have different behavior for indenting an empty line depending on context, so inside my indentation rules function, I have a case like this:

(`(:elem . empty-line-token)
     (if (smie-rule-parent-p ",") "," "---"))

(The "---" is a token that's always indented at column 0.)

The source language has definitions that look like this:

type Network = DC { reference_id    : ReferenceId
                  , inventory_class : InventoryClass
                  , children        : [Network]
                  }

when I start a new line in the middle of the { .. } block, I want it indented in line with the commas:

type Network = DC { reference_id    : ReferenceId
                  , inventory_class : InventoryClass
                  , children        : [Network]
                  ^ -- I want the cursor here after indenting.
                  }

The rule using smie-rule-parent-p "," accomplishes this, but it also fires in other parts of the code. In fact, it even triggers when there isn't a single comma in the whole buffer.

How does SMIE determine what the parent of a token is? Can I use that to indent empty lines differently depending on context like I'm trying here?