back1.0 bk1.0
back number 
The turtle moves backward by number steps. (If number is negative, the turtle moves forward.)
Turtles using this primitive can move a maximum of one unit per time increment. So bk 0.5 and bk 1 both take one unit of time, but bk 3 takes three.
If the turtle cannot move backward number steps because it is not permitted by the current topology the turtle will complete as many steps of 1 as it can and stop.