s <- "abcdefgh"
s.0
=> "a"

# starting from n characters in and of m length;
def (substr s start len)
  (s start start+len)
(substr s 3 2)
=> "de"

# starting from n characters in, up to the end of the string
(s 3 nil)
=> "defgh"

# whole string minus last character;
(s 3 -1)
=> "defg"

# starting from a known character within the string and of <tt>m</tt> length;
# starting from a known substring within the string and of <tt>m</tt> length.
let start (pos s pat)
  (s start start+m)