The reverse of tech_from_spec(): slot tables become row lists (all-NA
columns and empty tables are omitted; NA dimension values are omitted
from rows, i.e. read back as "applies to ALL").
The reverse of tech_from_spec(): slot tables become row lists (all-NA
columns and empty tables are omitted; NA dimension values are omitted
from rows, i.e. read back as "applies to ALL").