Nuclides are parts of the stylesheet source that are not directly useable on their own. This could be mixins, variables like colors and sizes, or settings.
To mark a variable or mixin as a nuclide, annotate it with the @nuclide tag, followed by the name of the nuclide.
Special forms are @color and @mixin.
@include base-font-family(600); @include base-font-family;