returns the size of a sequence
(length seq) (length! seq)
length function will return the number of elements in a sequence.
If the sequence is lazily computed, asynchronous, or otherwise incapable of returning a count, this function will raise an error. In order to perform a brute-force count of a sequence, use the
length! function, keeping in mind that it may never return a result.
(length '(1 2 3 4))
This example will return 4.