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$stylesheet: 'style';

@import 'rem-function';

.element {
  margin: rem(20);  
}

$base-font-size: 16;

@function rem ($pixels) {
  @if $stylesheet == 'ie' {
    @return $pixels + px;
  }
  else {
    @return ($pixels / $base-font-size) + rem;
  }
}

$stylesheet: 'ie';

@import 'rem-function';

.element {
  margin: rem(20);  
}