(Some of these are themselves predicates.)
some(is_prime, range(10,20)) ==> 1
some(is_prime, range(8,10)) ==> 0
Complementary to some is no
keep(P,L) = the list of items in L satisfying P
drop(P,L) = the list of items in L not satisfying P
find(P,L) = the suffix of L beginning with the first item satisfying P
If there is no such suffix, the result is
[ ].
find is similar to keep, except keep locates all items
Use first after find, to get actual item.