.cm-line {
	&:has(> br:only-child) {
		/* .cm-line with only one child of type <br> */
		@extend %empty-line !optional;
	}

	> br:only-child {
		/* placeholder on the only-child <br> inside of a .cm-line, so that it works inside of a :has() pseudo-selector */
		@extend %empty-line-br !optional;
	}

	&:where(.HyperMD-header) {
		@extend %heading !optional;
	}

	&:where(.HyperMD-quote.HyperMD-quote-1) {
		@extend %blockquote !optional;

		&:where(%empty-line + *) {
			/* blockquote preceeded by an empty line */
			@extend %blockquote-first-line !optional;
		}

		&:has(+ %empty-line-br) {
			/* here you have to use %empty-line-br to prevent a nested :has() pseudo-selector */
			/* blockquote followed by an empty line */
			@extend %blockquote-last-line !optional;
		}

		&:where(%empty-line + *):has(+ %empty-line-br) {
			/* again, %empty-line-br to prevent a nested :has() pseudo-selector */
			/* single-line blockquote */
			@extend %blockquote-single-line !optional;
			color: gold;

			/* 
                An overline has to match all conditions:
                1. is a blockquote (`.cm-line.HyperMD-quote.HyperMD-quote-1`)
                2. has adjacent empty lines before AND after (single-line blockquote)
                3. has a heading after its succeeding empty line.
            */
			&:has(+ * + %heading) {
				/* overline */
				@extend %overline !optional;
				color: lime;

				/* 
                    Selecting the empty line between the overline and the heading is relatively simple, as it can be selected as a sibling of %overline.
                    > `:where(%overline) + :where(%empty-line):has(+ %heading)`
                */
				+ %empty-line:has(+ %heading) {
					/* empty line following an overline */
					@extend %overline-empty-line !optional;
					background-color: lime;
				}
			}

            /*
                A subtitle has to match all conditions:
                1. is a blockquote (`.cm-line.HyperMD-quote.HyperMD-quote-1`)
                2. has adjacent empty lines before AND after (single-line blockquote)
                3. has a heading before its preceding empty line.

                How do I select the empty line between the subtitle and the heading????

                > `:where(%heading) + :where(%empty-line):has(+ %subtitle)`
                ^ this does not work because the %subtitle placeholder in and of itself uses :has() to ensure it has a following empty line.

                I can select any empty line after a heading, followed by a blockquote as such:
                > `:where(%heading) + :where(%empty-line):has(+ %blockquote)`
                But this isn't the desired result, as this would also select an empty line between a heading and a blockquote that is NOT a subtitle (multi-line blockquote).
            */
			&:where(%heading + * + *) {
				/* subtitle */
				@extend %subtitle !optional;
				color: cyan;
			}
		}

		+ %empty-line:has(+ %heading) {
			/* empty line following an overline */
			@extend %overline-empty-line !optional;
			background-color: lime;
		}
	}
}