-   Some way to control files with a variable number of columns, e.g.

    foo,bar,baz,quux
    flim,flam,flom

If I want the above parsed into just three fields, would I want:

    foo,bar,[baz,quux]
    flim,flam,[flom]

Or just:

    foo,bar,baz
    flim,flam,flom

How would I specify this in the API?
