Combination Referentials

[chromonym]

In Ithkuil IV, it is possible for any referential slot to have more than one actual referential.
That is not currently possible in Ithkapp.

(Potentially repurpose affix OptionBox code?)

Edit: this has been implemented, check first comment to see what I still need to do

[chromonym]

From commit ff488b6:

Note that I still need to:

• figure out why voicedness isn't being checked for cluster permissability
• disallow repeated referentials in the same cluster
• make the gloss code for secondary referentials reflective of the word