what to do with "unknown" variables

[Jos Theelen]

Suppose I have some variable that I want to use as an input for a 
SpatialPooler. The variable can be one out of 20 categories, while the 
categories have no connection to each other. In that case it seems 
reasonable to take 20 bits and give each category one bit.

But sometimes there are records, where the category is not known or not 
available. What would be a good solution? Take 21 bits and take 
“unknown” as a different and new category? Or take 20 bits and put them 
all on 0?

Jos Theelen