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$screen-xs-min: 767px;
$screen-sm-min: 768px;
$screen-md-min: 992px;
$screen-lg-min: 1200px;
@mixin media($class) {
  @if $class == xs {
    @media (max-width: $screen-xs-min) { @content; }
  }

  @else if $class == sm {
    @media (min-width: $screen-sm-min) { @content; }
  }

  @else if $class == md {
    @media (min-width: $screen-md-min) { @content; }
  }

  @else if $class == lg {
    @media (min-width: $screen-lg-min) { @content; }
  }

  @else {
    @warn "Breakpoint mixin supports: xs, sm, md, lg";
  }
}



//пример
footer {
  display: none;
  @include media(sm) {
    display: block;
  }
}