after class (based on Nick Belov's idea and top of p. 49 of Robin Wilson's book)
#AntiPC(P): inputs a list of length n-2 with entries from {1, ...,n} outputs the tree with
#labels {1, ...,n} whose Pruffer code is P
AntiPC:=proc(P) local n,V,i,P1,b1,E:
n:=nops(P)+2:
V:={seq(i,i=1..n)}:
P1:=P:
E:={}:

while P1<>[] do
b1:=min (V minus convert(P1,set)):
E:=E union {{P1[1],b1}}:
P1:=[op(2..nops(P1),P1)]:
V:=V minus {b1}:
od:

E union {V}:


end: