new_contents: from __future__ import absolute_import, division, print_function
  
  import tables as tb
  from blaze.expr.table import *
  from datashape import Record
  from ..dispatch import dispatch
  
  
  @dispatch(tb.Table)
  def discover(t):
      return t.shape[0] * Record([[col, t.coltypes[col]] for col in t.colnames])
  
  
  @dispatch(Selection, tb.Table)
  def compute_one(sel, t):
      s = eval_str(sel.predicate)
      return t.read_where(s)
  
  
  @dispatch(Head, tb.Table)
  def compute_one(h, t):
      return t[:h.n]
  
subject: add pytables compute draft