Marc Feeley writes:
It is a bad idea for the fill parameter of vector-copy to have a default. When fill is absent, it should be an error when start and end are not within the bounds of the sequence. Otherwise, some index calculation errors (off-by-one on end) may go unnoticed. Moreover, when it is supplied, fill should also be used when start is less than 0, for consistency with the case where end is greater to the length of the sequence.