Less - Issue with extend

Sorry for my english, don't hesitate to tell me if some sentences does not make sense. I'm working on a little less library for my website but i'm stuck on this. I try to make some usefull class for the padding and i think the way i try to write it can't work but i don't know how to do it properly. This is one part of my code :

    .padding {
        &-top {
            &-lg { padding-top: @lg_padding; }
        }
        &-bottom {
            &-lg { padding-bottom: @lg_padding; }
        }
        &-height {
            &-lg { &:extend(.padding-top-lg, .padding-bottom-lg); }
        }
    }

In reality i got left, right, top, bottom, height, width, and multiple size but it's just to show the idea. I know the syntaxe is not that bad cause it works if i try to use some other class in the extend, but with this kind of class ( sorry i don't know the name of class created with the '&' ) it's look like it's not possible.

Do somebody have advice to give me about it ? Thanks