如何在Snakemake中创建嵌套循环？

A = [1, 2, 3]
B = [4, 5, 6]

rule all:
    input:
        expand("output/{a}_{b}.txt", a=A, b=B)

rule generate_tasks:
    output:
        "tasks.txt"
    run:
        with open(output[0], "w") as f:
            for a, b in product(A, B):
                f.write(f"{a}\t{b}\n")

rule all:
    input:
        expand("output/{a}_{b}.txt", a=A, b=B)

rule generate_tasks:
    output:
        "tasks.txt"
    run:
        with open(output[0], "w") as f:
            for a, b in product(A, B):
                f.write(f"{a}\t{b}\n")

include: "tasks.txt"

rule process_task:
    input:
        task_file="tasks.txt"
    output:
        "output/{a}_{b}.txt"
    params:
        a=lambda wildcards: wildcards.task.split("\t")[0],
        b=lambda wildcards: wildcards.task.split("\t")[1]
    run:
        a = params.a
        b = params.b
        # 执行任务的代码
        shell("echo {a} {b} > {output}")