&key
from-end start end key test test-not → n &key
from-end start end key → n &key
from-end start end key → n 0
and nil, respectively.count, count-if, and count-if-not count and return the number of elements in the sequence bounded by start and end that satisfy the test.
The from-end has no direct effect on the result. However, if from-end is true, the elements of sequence will be supplied as arguments to the test, test-not, and key in reverse order, which may change the side-effects, if any, of those functions.
(count #\a "how many A's are there in here?")
→2
(count-if-not #'oddp '((1) (2) (3) (4)) :key #'car)
→2
(count-if #'upper-case-p "The Crying of Lot 49" :start 4)
→2
None.
None.
Should be prepared to signal an error of type type-error if sequence is not a proper sequence.