rule all:
    input:
        "file_b.txt"

rule file_a:
    output:
        "file_a.txt"
    run:
        import time

        with open(output[0], "w") as f:
            f.write("A" + str(time.time_ns()))

rule file_b:
    input:
        "file_a.txt"
    output:
        "file_b.txt"
    run:
        import time

        with open(output[0], "w") as f:
            f.write("D" + str(time.time_ns()))
