A single predicate function, a formula describing such a
predicate function, or a logical vector of the same length as .x.
Alternatively, if the elements of .x are themselves lists of
objects, a string indicating the name of a logical element in the
inner lists. Only those elements where .p evaluates to
TRUE will be modified.
x <- transpose(list(x = 1:10, y = rbernoulli(10)))
x %>% keep("y") %>% length()
x %>% keep(negate("y")) %>% length()
# Same asx %>% discard("y") %>% length()