peg b = the intermediary peg
peg c = the goal peg, where all disks have to be in the same order

constraints :
one disk at a time
not a big disk on a little one 

Furthermore if necessary I use to characterize the position of disks with
the variables below:

- sectors : the triangle (which represents the problem space) is divided in
four sectors
S1	top right Side
S2	top left Side
S3	bottom left Side
S4	bottom right Side

-goals : 
G4	disk 4 moves to peg C
G3	disk 3 moves to the place from which disk 4 can m