from turtle import *
replacements = {"A":"+BF-AFA-FB+",
                "B":"-AF+BFB+FA-",
                "+":"+",
                "-":"-",
                "F":"F",}
def curve_sequence(n):
    if n==0:
        yield "A"
        return
    for t in curve_sequence(n-1):
        yield from replacements[t]
speed(0)
hideturtle()
for move in curve_sequence(15):
    if move == "+":
        left(90)
    elif move == "-":
        right(90)
    elif move == "F":
        forward(5)