pub fn add_adjacent(&mut self, literal: TrimmedLiteral) -> Result<(), TrimmedLiteral> {
        if literal.variant().category() != self.variant.category() {
            log::debug!(
                "Adjacent literal is not the same comment variant: {:?} vs {:?}",
                literal.variant().category(),
                self.variant.category()
            );
            return Err(literal);
        }
        let previous_line = literal.span().end.line;
        if previous_line == self.coverage.1 + 1 {
            self.coverage.1 += 1;
            self.literals.push(literal);
            return Ok(());
        }

        let next_line = literal.span().start.line;
        if next_line + 1 == self.coverage.0 {
            self.literals.push(literal);
            self.coverage.1 -= 1;
            return Ok(());
        }

        Err(literal)
    }
Add a literal to a literal set, if the previous lines literal already
exists.

Returns literal within the Err variant if not adjacent
doc-chunks/src/literalset.rs