   n   internals   externals   externals - internals
    0           0           1                       1
    1           1           2                       1
    2           2           3                       1
    3           3           4                       1
    4           4           5                       1
    5           5           6                       1
   10          10          11                       1
   50          50          51                       1
 1000        1000        1001                       1

20000 random trees checked: X(n) = n + 1 holds in every case
