how to enable burst in PPPOE

i can’t find where to set burst limit for PPPOE user
here’s an export of profile and secrets

/ ppp profile
set default name="default" local-address=1.2.3.1 use-compression=default \
    use-vj-compression=default use-encryption=default only-one=default \
    change-tcp-mss=yes comment=""
add name="ClassA" local-address=1.2.3.1 use-compression=default \
    use-vj-compression=default use-encryption=default only-one=default \
    change-tcp-mss=default rate-limit=12k/80k comment=""
add name="ClassL" local-address=1.2.3.1 use-compression=default \
    use-vj-compression=default use-encryption=default only-one=default \
    change-tcp-mss=default rate-limit=8k/64k comment=""
add name="ClassC" local-address=1.2.3.1 use-compression=default \
    use-vj-compression=default use-encryption=default only-one=default \
    change-tcp-mss=default rate-limit=10k/64k comment=""
set default-encryption name="default-encryption" use-compression=default \
    use-vj-compression=default use-encryption=yes only-one=default \
    change-tcp-mss=yes comment=""



/ ppp secret
add name="userx" service=pppoe caller-id="" password="passx" profile=ClassA \
    remote-address=1.2.3.4 routes="" limit-bytes-in=0 limit-bytes-out=0 \
    comment="" disabled=no
add name="usery" service=pppoe caller-id="" password="passy" profile=ClassC \
    remote-address=1.2.3.5 routes="" limit-bytes-in=0 limit-bytes-out=0 \
    comment="" disabled=no

nothing about burst !!
i readed the manual i notice that rate limit can be a long string in which can set a value for burst

rate-limit (text; default: "") - rate limitation in form of rx-rate[/tx-rate] [rx-burst-rate[/tx-burst-rate]
[rx-burst-threshold[/tx-burst-threshold] [rx-burst-time[/tx-burst-time] [priority]
[rx-rate-min[/tx-rate-min]]]]

but i need an example as i tried and got no such argument

rate-limit="10k/64k 100k/640k 8k/50k 10"