we sometimes refer to a string by "word".
to concatenate a string with itself many times, we use the superscript notation to mean
the lexicographic order of strings is the same as the familiar dictionary order.
shortlex order or simply string order, is identical to lexicographic order, except that shorter strings precede longer strings. thus the string ordering of all strings over the alphabet is .
we use the notation to denote the number of times the character occurs in a string .
let be words, then