record node( data, ltree, rtree )

procedure main()
   local line, tree
   write("type tree in prefix form  eg. 1(2(3,4),5)")
   while line := read() do
    { tree := tform(line)
      write("tree walk")
      every write(walk(tree))     # walk generates nodes in postfix order
      write("leaves")
      every write(leaves(tree))   # leaves generates leaf nodes only
    }
end

procedure tform(s)           # parse (prefix) binary tree, eg. 1(2(3,4),5(6,7))
   local value, left, right
   if /s then return
   s ? if value := tab( upto( '(' ) ) then
        { move(1)
          left := tab( bal( ',' ) )
          move(1)
          right := tab( bal( ')' ) )
          return node( value, tform(left), tform(right) )
        }
       else return node(s)
end

procedure walk(t)                        # postfix
   suspend walk( \t.ltree | \t.rtree )   # generate left, then right
   return t.data                         # then data in t
end

procedure leaves(t)
   if not( \t.ltree | \t.rtree ) then return t.data  # t is a leaf
   suspend leaves( \t.ltree | \t.rtree )             # t not a leaf
end

#\fB Binary Trees. \fP