This function is similar to length-repeat-i in that a length value can be selected and all occurences of that value be processed. Here, the processing changes all selected values to corresponding length-rests.
(length-rests-i '1/16 '(1/16 1/8 1/16 1/16 1/16))
--> (-1/16 1/8 -1/16 -1/16 -1/16)
(length-rests-i 48 '(48 48 24))
--> (-48 -48 24)
In combination with other length functions this function has the potential for creating most effective rhythmic polyphonies.