$breakpoints: (xs: 576px,
  sm: 768px,
  md: 992px,
  lg: 1200px);

@mixin respond-above($breakpoint) {
  @if map-has-key($breakpoints, $breakpoint) {
    $breakpoint-value: map-get($breakpoints, $breakpoint);

    @media (min-width: $breakpoint-value) {
      @content;
    }
  }

  @else {
    @warn 'Invalid breakpoint: #{$breakpoint}.';
  }
}

@mixin respond-below($breakpoint) {
  @if map-has-key($breakpoints, $breakpoint) {
    $breakpoint-value: map-get($breakpoints, $breakpoint);

    @media (max-width: ($breakpoint-value - 1)) {
      @content;
    }
  }

  @else {
    @warn 'Invalid breakpoint: #{$breakpoint}.';
  }
}

@mixin respond-between($lower, $upper) {
  @if map-has-key($breakpoints, $lower) and map-has-key($breakpoints, $upper) {
    $lower-breakpoint: map-get($breakpoints, $lower);
    $upper-breakpoint: map-get($breakpoints, $upper);

    @media (min-width: $lower-breakpoint) and (max-width: ($upper-breakpoint - 1)) {
      @content;
    }
  }

  @else {
    @if (map-has-key($breakpoints, $lower)==false) {
      @warn 'Your lower breakpoint was invalid: #{$lower}.';
    }

    @if (map-has-key($breakpoints, $upper)==false) {
      @warn 'Your upper breakpoint was invalid: #{$upper}.';
    }
  }
}

@mixin border-radius($px: 0) {
  -webkit-border-radius: $px;
  -moz-border-radius: $px;
  border-radius: $px;
}

@mixin box-shadow($value) {
  -webkit-box-shadow: $value;
  -moz-box-shadow: $value;
  box-shadow: $value;
}

@mixin transition($element: all, $time: .5s, $option: ease) {
  -webkit-transition: $element $time $option;
  transition: $element $time $option;
}

@mixin opacity($value) {
  $IEValue: $value*100;
  opacity: $value;
  -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity="+$IEValue+")";
  filter: alpha(opacity=$IEValue);
}

@mixin flexbox() {
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
}

@mixin flex($values) {
  -webkit-box-flex: $values;
  -moz-box-flex: $values;
  -webkit-flex: $values;
  -ms-flex: $values;
  flex: $values;
}

@mixin flex-direction($direction) {
  -webkit-flex-direction: $direction;
  -moz-flex-direction: $direction;
  -ms-flex-direction: $direction;
  flex-direction: $direction;
}

@mixin flex-wrap($wrap) {
  -webkit-flex-wrap: $wrap;
  -moz-flex-wrap: $wrap;
  -ms-flex-wrap: $wrap;
  flex-wrap: $wrap;
}

@mixin flex-flow($flow) {
  -webkit-flex-flow: $flow;
  -moz-flex-flow: $flow;
  -ms-flex-flow: $flow;
  flex-flow: $flow;
}

@mixin order($val) {
  -webkit-box-ordinal-group: $val;
  -moz-box-ordinal-group: $val;
  -ms-flex-order: $val;
  -webkit-order: $val;
  order: $val;
}

@mixin flex-grow($grow) {
  -webkit-flex-grow: $grow;
  -moz-flex-grow: $grow;
  -ms-flex-grow: $grow;
  flex-grow: $grow;
}

@mixin flex-shrink($shrink) {
  -webkit-flex-shrink: $shrink;
  -moz-flex-shrink: $shrink;
  -ms-flex-shrink: $shrink;
  flex-shrink: $shrink;
}

@mixin flex-basis($width) {
  -webkit-flex-basis: $width;
  -moz-flex-basis: $width;
  -ms-flex-basis: $width;
  flex-basis: $width;
}

@mixin justify-content($justify) {
  -webkit-justify-content: $justify;
  -moz-justify-content: $justify;
  -ms-justify-content: $justify;
  justify-content: $justify;
  -ms-flex-pack: $justify;
}

@mixin align-content($align) {
  -webkit-align-content: $align;
  -moz-align-content: $align;
  -ms-align-content: $align;
  align-content: $align;
}

@mixin align-items($align) {
  -webkit-align-items: $align;
  -moz-align-items: $align;
  -ms-align-items: $align;
  align-items: $align;
}

@mixin align-self($align) {
  -webkit-align-self: $align;
  -moz-align-self: $align;
  -ms-align-self: $align;
  align-self: $align;
}