// Returns E^x, where x is the argument, and E is Euler's constant, the base of the natural logarithms.
// @param {Number} $x
// @example
//     exp(1)  // 2.71828
//     exp(-1) // 0.36788
@function exp ($x) {
    $ret: 0;
    @for $n from 0 to 24 {
        $ret: $ret + pow($x, $n) / fact($n);
    }
    @return $ret;
}