2 |
18. s(x,n) s(x+1,n)
19. s(y-1,n) s(y,n)
20. k:{1...n}. Moving disk k of n takes s(x) to s(x+1)
21. k:{1...n}. Moving disk k of n takes s(y-1) to s(y)
p',q':Peg.
( u:{a...x}. s(u,n) = p) & ( u:{y...z}. s(u,n) = q)
& s(x) = ( i.p') {1...n-1} Peg & s(y) = ( i.q') {1...n-1} Peg
& p p'
& q q'
 | 15 steps |