这段指令跑出来是多少

一段四条指令的小程序，和一个十来行的解释器：

PROG = [
    ("SET",   "x", 3),
    ("ADD",   "x", 4),
    ("MUL",   "x", 3),
    ("PRINT", "x", None),
]

def run(prog):
    env, out = {}, []
    for op, name, arg in prog:
        if op == "SET":
            env[name] = arg
        elif op == "ADD":
            env[name] = env[name] + arg
        elif op == "MUL":
            env[name] = env[name] * arg
        elif op == "PRINT":
            out.append(env[name])
    return out

print("/".join(str(x) for x in run(PROG)))