Clearfix mixin generating output everytime mixins.less is imported

[Tauren Mills]

I've notice that every time I import mixins.less, .clearfix is defined in
the output CSS. This is how it is currently defined:

// Clearfix
// --------
// For clearing floats like a boss h5bp.com/q
.clearfix {
  *zoom: 1;
  &:before,
  &:after {
    display: table;
    content: "";
  }
  &:after {
    clear: both;
  }
}

Is this the intended behavior, or is it a mistake? Should it be made into a
function by adding parenthesis?

// Clearfix
// --------
// For clearing floats like a boss h5bp.com/q
.clearfix() {
  *zoom: 1;
  &:before,
  &:after {
    display: table;
    content: "";
  }
  &:after {
    clear: both;
  }
}

Bootstrap seems to still work fine when I make the change above. Since
mixins.less might be included multiple times in different files, it would
be nice if everything in it would only create output when explicitly called.

I'd be happy to submit a pull request if this is a mistake. If it is
intended, what is the best way to avoid having multiple .clearfix
declarations?

Thanks,
Tauren