Error: Base-level rules cannot contain the parent-selector-referencing character '&'.
        on line 6 of /sass/spec/libsass-issues/issue_1644/mixin-parent/input.scss, in `@content'
        from line 2 of /sass/spec/libsass-issues/issue_1644/mixin-parent/input.scss, in `parent'
        from line 5 of /sass/spec/libsass-issues/issue_1644/mixin-parent/input.scss
  Use --trace for backtrace.
