Sass: To abbreviate or not to abbreviate

@incuna/frontend 

I'd like to standardise our use of Sass shorthands, as I've seen it inconsistent across projects, and this is language syntax issue, rather than a file formatting issue. 

e.g. whether to use:
`+mixin_name` instead of `@include mixin_name`
`=mixin_name` instead of `@mixin mixin_name`
Docs: http://sass-lang.com/docs/yardoc/file.INDENTED_SYNTAX.html

Some time back I think we agreed to always use the long form, but this wasn't put in the style guide (and I think before @henrahmagix and @rogerhutchings started).

Personally, I'm strongly against the shorthand for the following reasons:
1) I do not think they are intuitive or self-descriptive. `@extend` and `@include` describe what they are doing clearly, and I think the shorthands won't be clear to anyone who is not very familiar with sass (e.g. backend, freelancers, new employees). 
2) I also think the symbols used are confusing in the context of CSS and selectors, which have a number of similar and potentially confusing symbols already. 
e.g.
```
#some-id + sibling-selector
    +mixin
```
Uses the same `+` character for different operations.



However, I am open to other thoughts. Does anyone have any good arguments for wanting to use the shorthands? Or can I add **always use the long form** it to the style guide?