fade($color, $percent) 
    rgba($color, $percent)



placeholder($color = $input-placeholder-color)
    // Firefox
    &::-moz-placeholder
        color: $color;
        opacity: 1; // Override Firefox's unusual default opacity; see https://github.com/twbs/bootstrap/pull/11526
    // Internet Explorer 10+
    &:-ms-input-placeholder
        color: $color
    // Safari and Chrome
    &::-webkit-input-placeholder
        color: $color

// for select and input like component's arrow
inner-arrow()
    position: absolute
    top: 50%
    right: 8px
    line-height: 1
    margin-top: -7px
    font-size: $font-size-base
    color: $subsidiary-color
    transition: all $transition-time $ease-in-out


hover($color = $input-hover-border-color)
    border-color: tint($color, 20%)