==> series/series.19.p <==
0 01 01011 0101101011011 0101101011011010110101101101011011 etc.
Each string is formed from the previous string by substituting '01' for '0'
and '011' for '1' simultaneously at each occurance.
Notice that each string is an initial substring of the previous string so
that we may consider them all as initial substrings of an infinite string.
The puzzle then is, given n, determine if the nth digit is 0 or 1 without
having to construct all the previous digits. That is, give a non-recursive
formula for the nth digit.