// Returns the arcsine of a number.
// @param {Number} $x A number between -1 and 1.
// @example
//     asin(0.1) // 0.10017
//     asin(-1)  // -1.5708
@function asin ($x) {
    @if $x > 1 or $x < -1 {
        @warn "Argument for `asin()` must be a number between -1 and 1";
        @return null;
    }
    @return atan($x / sqrt(1 - $x * $x));
}