Can anyone explain to me
 why the ** in "**20** lines"?
 why the $ in "length $n$"?
 which other path than 102030407090 is possible in the sample program?
 what will say "the number of paths ... is the sequence 0,2,2,...?" Isn't the number dependent of a,b? Does 0 make sense?
Perhaps I'm the only one who doesn't understand this.
