adamgiese

@mixin valid-quantity($quantity) {
  @if type-of($quantity) != 'number' {
    @error 'The "quantity" parameter must be a number!';
  }
  @if not(unitless($quantity)) {
    @error 'The "quantity" parameter must not have a unit!';
  }
  @if $quantity < 0 {
    @error 'The "quantity" parameter must be at least 0!';
  }