code: def vstack(xs):
      """Concatenate variables vertically (row wise).
  
      Args:
          xs (list of :class:`~chainer.Variable` or :ref:`ndarray`):
              Input variables to be concatenated. The variables must have the
              same ``ndim``. When the variables have the second axis (i.e.
              :math:`ndim \\geq 2`), the variables must have the same shape
              along all but the first axis. When the variables do not have the
              second axis(i.e. :math:`ndim < 2`), the variables must have the
              same shape.
  
      Returns:
          ~chainer.Variable:
              Output variable. When the input variables have the second axis
              (i.e. :math:`ndim \\geq 2`), the shapes of inputs and output are
              the same along all but the first axis. The length of first axis
              is the sum of the lengths of inputs' first axis.
              When the variables do not have the second axis (i.e.
              :math:`ndim < 2`), the shape of output is ``(2, N)`` (``N`` is the
              size of the input variable).
  
      .. admonition:: Example
  
          >>> x1 = np.array((1, 2, 3))
          >>> x1.shape
          (3,)
          >>> x2 = np.array((2, 3, 4))
          >>> x2.shape
          (3,)
          >>> y = F.vstack((x1, x2))
          >>> y.shape
          (2, 3)
          >>> y.array
          array([[1, 2, 3],
                 [2, 3, 4]])
          >>> x1 = np.arange(0, 12).reshape(3, 4)
          >>> x1.shape
          (3, 4)
          >>> x1
          array([[ 0,  1,  2,  3],
                 [ 4,  5,  6,  7],
                 [ 8,  9, 10, 11]])
          >>> x2 = np.arange(12, 20).reshape(2, 4)
          >>> x2.shape
          (2, 4)
          >>> x2
          array([[12, 13, 14, 15],
                 [16, 17, 18, 19]])
          >>> y = F.vstack([x1, x2])
          >>> y.shape
          (5, 4)
          >>> y.array
          array([[ 0,  1,  2,  3],
                 [ 4,  5,  6,  7],
                 [ 8,  9, 10, 11],
                 [12, 13, 14, 15],
                 [16, 17, 18, 19]])
  
      """
  
      return Vstack().apply((xs))[0]
  
file_path: chainer/chainer/functions/array/vstack.py
imports: import numpy
  import six
  import chainer
  from chainer import backend
  from chainer import function_node
  from chainer.utils import type_check
label_desc: Use image node_id 1 for calling a global function with example usage: vstack(xs) and returns: Vstack