lm:k-admissibility: fix induction

I was being blind with this, we can't have induction on k.
But the mix wasn't good either, and the lemma won't work for k=0 anyway.

Now the induction is purely on the size of the tree,
and the lemma requires k>0.

None of this affected the fact that, with our definition,
paths *are* 1-admissible -- they do have at most one child
and the child is 1-admissible as well. I'm very sure of this.