.desktop-and-old-ie(@rules) {
    @media screen and (min-width: 1200) { @rules(); }
    html.lt-ie9 &                       { @rules(); }
}

header {
    background-color: blue;

    .desktop-and-old-ie({
        background-color: red;
    });
}

.e(@name, @rules) {
    &__@{name} { @rules(); }
}

.m(@name, @rules) {
    &--@{name} { @rules(); }
}

.btn {
    .e(span, {  // .btn__span
        display: inline-block;
    });
    .m(primary, { // .btn--primary
        background: blue;
    });
}