// Convert to unitless rad
@function unitless-rad ($angle) {
    @if unitless($angle) {
        @return $angle;
    } @else if unit($angle) == rad {
        @return $angle / 1rad;
    } @else if unit($angle) == deg {
        @return deg-to-rad($angle, false);
    } @else if type-of($angle) != number or not unitless($angle) {
        @warn "#{ $angle } is not a valid number.";
        @return $angle;
    }
}