def create_null_matrix(self, row, col):
		""" Create a matrix using the null value

		This is a private function called by __init__.
		"""
		if not row > 0:
			raise ValueError("invalid number of rows %d" % row)
		if not col > 0:
			raise ValueError("invalid number of columns %d" % col)
		if row == 1 and col == 1:
			raise ValueError("Cannot create 1x1 matrix")
		# Note, you cannot simply write
		#	self.m = [[self.null_element]*col]*row
		# because this will make all the rows references of a single instance.
		self.m = []
		for i in xrange(row):
			self.m.append([])
			for j in xrange(col):
				self.m[i].append(self.null_element)
 Create a matrix using the null value

		This is a private function called by __init__.
		
recipes/Python/189971_Basic_Linear_Algebra_Matrix/recipe-189971.py