@function convert-angle($value, $unit) {
    $convertable-units: deg grad turn rad;
    $conversion-factors: 1 (10grad/9deg) (1turn/360deg) (3.1415926rad/180deg);

    @if index($convertable-units, unit($value)) and index($convertable-units, $unit) {
        @return $value / nth($conversion-factors, index($convertable-units, unit($value))) * nth($conversion-factors, index($convertable-units, $unit));
    }
    @warn "Cannot convert `#{unit($value)}` to `#{$unit}`.";
}

/// Test if '$value' is an angle
/// @param {*} $value - Value to test
/// @return {Bool}
@function is-direction($value) {
    $is-direction: index((to top, to top right, to right top, to right, to bottom right, to right bottom, to bottom, to bottom left, to left bottom, to left, to left top, to top left), $value);
    $is-angle: type-of($value) == 'number' and index('deg' 'grad' 'turn' 'rad', unit($value));
    @return $is-direction or $is-angle;
}

/// Convert a direction to legacy syntax
/// @param {Keyword | Angle} $value - Value to convert
/// @require {function} is-direction
/// @require {function} convert-angle
@function legacy-direction($value) {
    @if is-direction($value) == false {
        @warn "Cannot convert `#{$value}` to legacy syntax because it doesn't seem to be an angle or a direction";
    }
    $conversion-map: ( "to top" : "bottom", "to top right" : "bottom left", "to right top" : "left bottom", "to right" : "left", "to bottom right" : "top left", "to right bottom" : "left top", "to bottom" : "top", "to bottom left" : "top right", "to left bottom" : "right top", "to left" : right, "to left top" : "right bottom", "to top left" : "bottom right" );

    @if map-has-key($conversion-map, $value) {
      @return map-get($conversion-map, $value);
    }
    @return 90deg - convert-angle($value, 'deg');
}

/// Mixin printing a linear-gradient
/// as well as a plain color fallback
/// and the `-webkit-` prefixed declaration
/// @access public
/// @param {String | List | Angle} $direction - Linear gradient direction
/// @param {Arglist} $color-stops - List of color-stops composing the gradient
@mixin linear_gradient($direction, $color-stops...) {
    @if is-direction($direction) == false {
        $color-stops: ($direction, $color-stops);
        $direction: 180deg;
    }
    background: nth(nth($color-stops, 1), 1);
    background: -webkit-linear-gradient(legacy-direction($direction), $color-stops);
    background: linear-gradient($direction, $color-stops);
}
@mixin radial_gradient($position: center, $shape: circle, $extent: farthest-corner, $color-stops...) {
    background: -moz-radial-gradient($position, $shape cover, $color-stops);
    background: -webkit-radial-gradient(center, $shape cover, $color-stops);
    background: -o-radial-gradient($position, $shape cover, $color-stops);
    background: -ms-radial-gradient($position, $shape cover, $color-stops);
    background: radial-gradient($shape $extent at $position, $color-stops);
    filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='nth(nth($color-stops, 1), 1)', endColorstr='nth(nth($color-stops, 2), 1)', GradientType=1);
}
@mixin box_shadow($thickness, $shadowcolor, $msequivcolor) {
    box-shadow: $thickness $shadowcolor;
    -webkit-box-shadow: $thickness $shadowcolor;
    -moz-box-shadow: $thickness $shadowcolor;
    // filter: progid:DXImageTransform.Microsoft.Shadow(direction=135,color='#{$msequivcolor}',strength=nth($thickness, 1));
}