def content_exists
      # frames.last is the stack frame for this function,
      # so we use frames[-2] to get the frame before that.
      mixin_frame = environment.stack.frames[-2]
      unless mixin_frame && mixin_frame.type == :mixin
        raise Sass::SyntaxError.new("Cannot call content-exists() except within a mixin.")
      end
      bool(!environment.caller.content.nil?)
    end
def content_exists
      # frames.last is the stack frame for this function,
      # so we use frames[-2] to get the frame before that.
      mixin_frame = environment.stack.frames[-2]
      unless mixin_frame && mixin_frame.type == :mixin
        raise Sass::SyntaxError.new("Cannot call content-exists() except within a mixin.")
      end
      bool(!environment.caller.content.nil?)
    end
Check whether a mixin was passed a content block.

 Unless `content-exists()` is called directly from a mixin, an error will be raised.

 @example
   @mixin needs-content {
     @if not content-exists() {
       @error "You must pass a content block!"
     }
     @content;
   }

 @overload content_exists()
 @return [Sass::Script::Value::Bool] Whether a content block was passed to the mixin.