// Returns the arccosine of a number.
// @param {Number} $x A number between -1 and 1.
// @example
//     acos(0.1) // 1.47063
//     acos(-1)  // 3.14159
@function acos ($x) {
    @if $x > 1 or $x < -1 {
        @warn "Argument for `acos()` must be a number between -1 and 1";
        @return null;
    }
    @return $PI / 2 - asin($x);
}