晋太元中，武陵人捕鱼为业。缘溪行，忘路之远近。忽逢桃花林，夹岸数百步，中无杂树，芳草鲜美，落英缤纷。渔人甚异之，复前行，欲穷其林。 　　林尽水源，便得一山，山有小口，仿佛若有光。便舍船，从口入。初极狭，才通人。复行数十步，豁然开朗。土地平旷，屋舍俨然，有良田、美池、桑竹之属。阡陌交通，鸡犬相闻。其中往来种作，男女衣着，悉如外人。黄发垂髫，并怡然自乐。 　　见渔人，乃大惊，问所从来。具答之。便要还家，设酒杀鸡作食。村中闻有此人，咸来问讯。自云先世避秦时乱，率妻子邑人来此绝境，不复出焉，遂与外人间隔。问今是何世，乃不知有汉，无论魏晋。此人一一为具言所闻，皆叹惋。余人各复延至其家，皆出酒食。停数日，辞去。此中人语云：“不足为外人道也。”(间隔 一作：隔绝) 　　既出，得其船，便扶向路，处处志之。及郡下，诣太守，说如此。太守即遣人随其往，寻向所志，遂迷，不复得路。 　　南阳刘子骥，高尚士也，闻之，欣然规往。未果，寻病终。后遂无问津者。 .
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Current File : //usr/lib64/python2.7/Demo/turtle/tdemo_lindenmayer_indian.py
#! /usr/bin/python2.7
"""       turtle-example-suite:

        xtx_lindenmayer_indian.py

Each morning women in Tamil Nadu, in southern
India, place designs, created by using rice
flour and known as kolam on the thresholds of
their homes.

These can be described by Lindenmayer systems,
which can easily be implemented with turtle
graphics and Python.

Two examples are shown here:
(1) the snake kolam
(2) anklets of Krishna

Taken from Marcia Ascher: Mathematics
Elsewhere, An Exploration of Ideas Across
Cultures

"""
################################
# Mini Lindenmayer tool
###############################

from turtle import *

def replace( seq, replacementRules, n ):
    for i in range(n):
        newseq = ""
        for element in seq:
            newseq = newseq + replacementRules.get(element,element)
        seq = newseq
    return seq

def draw( commands, rules ):
    for b in commands:
        try:
            rules[b]()
        except TypeError:
            try:
                draw(rules[b], rules)
            except:
                pass


def main():
    ################################
    # Example 1: Snake kolam
    ################################


    def r():
        right(45)

    def l():
        left(45)

    def f():
        forward(7.5)

    snake_rules = {"-":r, "+":l, "f":f, "b":"f+f+f--f--f+f+f"}
    snake_replacementRules = {"b": "b+f+b--f--b+f+b"}
    snake_start = "b--f--b--f"

    drawing = replace(snake_start, snake_replacementRules, 3)

    reset()
    speed(3)
    tracer(1,0)
    ht()
    up()
    backward(195)
    down()
    draw(drawing, snake_rules)

    from time import sleep
    sleep(3)

    ################################
    # Example 2: Anklets of Krishna
    ################################

    def A():
        color("red")
        circle(10,90)

    def B():
        from math import sqrt
        color("black")
        l = 5/sqrt(2)
        forward(l)
        circle(l, 270)
        forward(l)

    def F():
        color("green")
        forward(10)

    krishna_rules = {"a":A, "b":B, "f":F}
    krishna_replacementRules = {"a" : "afbfa", "b" : "afbfbfbfa" }
    krishna_start = "fbfbfbfb"

    reset()
    speed(0)
    tracer(3,0)
    ht()
    left(45)
    drawing = replace(krishna_start, krishna_replacementRules, 3)
    draw(drawing, krishna_rules)
    tracer(1)
    return "Done!"

if __name__=='__main__':
    msg = main()
    print msg
    mainloop()

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