'Move the turtle backward by distance.
Aliases: back | backward | bk
Argument:
distance -- a number
Move the turtle backward by distance ,opposite to the direction the
turtle is headed. Do not change the turtle\'s heading.
Example (for a Turtle instance named turtle):
>>> turtle.position()
(0.00, 0.00)
>>> turtle.backward(30)
>>> turtle.position()
(-30.00, 0.00)'

def back(self, distance):

    self._go((- distance))
