Relative font size Compass mixin

relative-font-size.scss

@mixin relative-font-size( $px-size, $unit: 'rem', $context: $base-font-size ) {                                                                                                                                                               
    @if $px-size == 0 {
        @error "$px-size value can't be equalse to 0.";
    }

    @if unit( $px-size ) != "px" {
        @error "$px-size must be value in pixels."
    }

    @if $unit != 'rem'
    and $unit != 'em'
    and $unit != '%' {
        @error "$unit must be 'rem' or 'em' or '%'.";
    }

    font-size: $px-size;
    font-size: $px-size / $context + 0#{$unit};
}