text: """
  Copyright 2013 Steven Diamond
  
  This file is part of CVXPY.
  
  CVXPY is free software: you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation, either version 3 of the License, or
  (at your option) any later version.
  
  CVXPY is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
  
  You should have received a copy of the GNU General Public License
  along with CVXPY.  If not, see <http://www.gnu.org/licenses/>.
  """
  
  import cvxpy.lin_ops.lin_utils as lu
  from cvxpy.atoms.affine.affine_atom import AffAtom
  import numpy as np
  
  
  def vstack(arg_list):
      """Wrapper on vstack to ensure list argument.
      """
      return Vstack(*arg_list)
  
  
  class Vstack(AffAtom):
      """ Vertical concatenation """
      # Returns the vstack of the values.
      @AffAtom.numpy_numeric
      def numeric(self, values):
          return np.vstack(values)
  
      # The shape is the common width and the sum of the heights.
      def shape_from_args(self):
          self.args[0].shape
          if self.args[0].ndim == 0:
              return (len(self.args), 1)
          elif self.args[0].ndim == 1:
              return (len(self.args), self.args[0].shape[0])
          else:
              rows = sum(arg.shape[0] for arg in self.args)
              return (rows,) + self.args[0].shape[1:]
  
      # All arguments must have the same width.
      def validate_arguments(self):
          model = self.args[0].shape
          for arg in self.args[1:]:
              if len(arg.shape) != len(model) or \
                 (len(model) > 1 and model[1:] != arg.shape[1:]) or \
                 (len(model) <= 1 and model != arg.shape):
                  raise ValueError(("All the input dimensions except"
                                    " for axis 0 must match exactly."))
  
      @staticmethod
      def graph_implementation(arg_objs, shape, data=None):
          """Stack the expressions vertically.
  
          Parameters
          ----------
          arg_objs : list
              LinExpr for each argument.
          shape : tuple
              The shape of the resulting expression.
          data :
              Additional data required by the atom.
  
          Returns
          -------
          tuple
              (LinOp for objective, list of constraints)
          """
          return (lu.vstack(arg_objs, shape), [])
  