That's very helpful. Shouldn't be too hard to match in R. The equivalent missing value in R would be `NULL`. 

At least to start I will keep the R-side biom class an extension of the list-class, with no automatic changes to the list, and then let the accessors worry about detecting `"null"` strings when they attempt to build tables. This means basic I/O will be purely JSON and accomplished with the RJSONIO-package's utilities written in C. Should be fast.