FILL
====

syntax: (fill <sequence> <element>
              [:start <start> :end <end>])

This function fills the sequence with an element.
Optionally you can set a start- and end-position.
FILL returns a modificated sequence and changes
the original!

example:       >(fill "abcdefg" #\z :start 1 :end 6)
               "azzzzzg"