framed.external.set

difference

(difference s1 & sets)
Construct a new ExternalSet that is the elements of the first set
without the elements of the following sets

union

(union s1 s2)
Construct a new ExternalSet that is the union of set-like s1 and s2
Taken from http://aphyr.github.io/tesser/tesser.core.html#var-fold

union'

(union' & sets)
Set union assuming all sets are ExternalSets; uses Riffle's
efficient merge procedure and returns a new ExternalSet