We should be able to compose by column AND nested table

[agmenc]

If I have a column (say: "message") and it expects nested tables (say: "From URL" and "From Disk") I want to be able to compose these ideas, such that it is easy to assign a message from a file on the disk, without using yucky callbacks.

I think this probably calls for a NestedTableProbe, which perhaps ought to declare the supported nestlings, and which returns a result that leaves the surrounding cell undecorated.