Make pst a parameter of grow.
Make digraph a parameter of grow.
Rename next -> next-f.
Remove let bindings from next-f and add comments.
Tighten the scope for intitial-{f,pst,digraph}.

Split these bindings out of the top-level let and move them to the
smallest scope possible, wrapping only the initial call to grow.
Rename L -> last-stree.