from turtle import *

def centered_circle(radius):
    """Draws a circle, centered on the turtle's current position.
    The built-in circle function annoyingly draws a circle whose center
    is one radius left of the turtle.
    """
    right(90)
    penup()
    forward(radius)
    pendown()
    left(90)
    circle(radius)
    left(90)
    penup()
    forward(radius)
    pendown()
    right(90)

def draw_boundary(distance):
    """Draws the boundary, a square `distance` from the origin on each side.
    """
    penup()
    goto(-distance, -distance)
    pendown()
    goto(distance, -distance)
    goto(distance, distance)
    goto(-distance, distance)
    goto(-distance, -distance)