Why is there a space leak here?

[Tom Pledger]

Michal Gajda writes:
 | On Tue, 29 May 2001, Tom Pledger wrote:
 :
 | > When you consume the (3N)th cell of v, you can't yet garbage collect
 | > the Nth cell because it will be needed for generating the (3N+1)th,
 | > (3N+2)th and (3N+3)th.
 | > 
 | > So, as you proceed along the list, about two thirds of it must be
 | > retained in memory.
 | 
 | Last sentence seems false. You free up Nth cell of v when you
 | finish with 3Nth cell of result.

I counted from 0.  Scouts' honour.  Call (!!) as a witness.

;-)