TODO

* Tables
 ** Tables has to be boxed. Since Lua permits this:
  a={}
  b=a --> b and a shared the same object
  a=nil --> b is nil too
  This may not be implemented with assoc-list in Scheme directly, a boxed one
  is an intuitive way to think about.

 ** You can assign nil to a table field to delete it, which is similar to hashtable
    in Scheme. So that's possibly one of the implementation.
    *** It's almost the same with table.remove, but not only set nil, additionally,
    it moves down other elements to close space and decrements the size of the
    array. When called without a position, it removes the last element of the
    array.
    I think hash-remove! is enough.
    But it's better the check the key for eq? eqv? equal? for better performance.

 ** If you use table as dictionary, say:
    a={name='john', age=15}
    print(#a) --> 0
    You can never get the size of table by # which is actually `table.getn'
    The only way is to use a loop and count it.    

    But if you use table as array, say:
    a={1,2,3}
    You can use # for it:
    print(#a) --> 3

    So this should be considered in the compile-tree-il.

* Assignment
  ** Every `(assign ...) node should be constrained to the current scope.


* Globals
  **  An assignment to a global variable x = val is equivalent to the assignment
      _ENV.x = val