@function be-less-than($this) {
  $actual: actual();
  $when: false;

  @if type-of($actual) == "number" {
    $when: $actual < $this;
  } @else {
    $when: error("type", "The `be-less-than` matcher is only for numbers");
  }

  @return should("be less than", $this, $when);
}