// k2 recursive

/ subtree (recursive)
st:{(./)._n,+{:[5=4:z;,/_f'[x,/:!z;y;z[]];y z;,(x;:;z);()]}[();y;x]}

/ pretty-print (recursive)
pp:{`0:{:[5=4:y;,/(x$!y){(,x),y}'(1+x)_f'y[];,(1+x)$y]}[1;x]}

\

/ tree
  t[`a;`b`c`d;`e`f]:3 2_draw 100

/ print t
  pp t
a
 b
  e
   31
  f
   51
 c
  e
   29
  f
   35
 d
  e
   17
  f
   89

/ subtree where 50<leaf
  s:st[t;50<]

/ print s
  pp s
a
 b
  f
   51
 d
  f
   89