p-tree

# a non-determ. automaton parsing binary trees
Ƥ(X×X)

a1: { (a2,a1), (a1,a2) }
a2: { (a1,a2) }
a3: { (a2,a1) }

b1: { (b2,b3), (b2,b2) }
b2: { }
b3: { }
b4: { (b3,b3) }


# a circle that collapses
bot: {}
c1: { (c2,bot) }
c2: { (c3,bot) }
c3: { (c4,bot) }
c4: { (c5,bot) }
c5: { (c6,bot) }
c6: { (c7,bot) }
c7: { (c8,bot) }
c8: { (c9,bot) }
c9: { (c10,bot) }
c10: { (c1,bot) }

# a circle that does not collapse
# because the last tuple is swapped
d1: { (d2,bot) }
d2: { (d3,bot) }
d3: { (d4,bot) }
d4: { (d5,bot) }
d5: { (d6,bot) }
d6: { (d7,bot) }
d7: { (d8,bot) }
d8: { (d9,bot) }
d9: { (d10,bot) }
d10: { (d1,d1) }