$grid-breakpoints: (
  xs: 0,
  sm: 576px,
  md: 768px,
  lg: 992px,
  xl: 1200px
) !default;

@include _assert-ascending($grid-breakpoints, "$grid-breakpoints"); // The parser broke down here
@include _assert-starts-at-zero($grid-breakpoints, "$grid-breakpoints");

$container-max-widths: (
  sm: 540px,
  md: 720px,
  lg: 960px,
  xl: 1140px
) !default;

@include _assert-ascending($container-max-widths, "$container-max-widths");

@mixin f {
	border-top: 0;
	border-right: $a solid transparent;  // previously it was considered a global variable
}