lisp/ox-latex.el: Add the `:literal' attribute to verse block.

* (org-latex-verse-block): Now the treatment of blank lines is
consistent with the syntax of the LaTeX `verse' environment, and the
one provided by the `verse' package.  If the `:literal' attribute is
used, all blank lines are preserved and exported as
`\vspace*{\baselineskip}', including the blank lines before or after
contents.  The rx code has been suggested by Ihor Radchenko, to
improve readability.

* doc/org-manual.org (Verse blocks in LaTeX export): The new feature
is documented.

* testing/lisp/test-ox-latex.el (test-ox-latex/verse): Updated.

doc/org-manual.org

@@ -14458,10 +14458,10 @@ The LaTeX export backend converts horizontal rules by the specified
 #+cindex: verse blocks, in @LaTeX{} export
 #+cindex: @samp{ATTR_LATEX}, keyword
 
-The LaTeX export backend accepts four attributes for verse blocks:
-=:lines=, =:center=, =:versewidth= and =:latexcode=.  The three first
-require the external LaTeX package =verse.sty=, which is an extension
-of the standard LaTeX environment.
+The LaTeX export backend accepts five attributes for verse blocks:
+=:lines=, =:center=, =:versewidth=, =:latexcode= and =:literal=.  The
+three first require the external LaTeX package =verse.sty=, which is
+an extension of the standard LaTeX environment.
 
 - =:lines= :: To add marginal verse numbering.  Its value is an
   integer, the sequence in which the verses should be numbered.
@@ -14473,6 +14473,16 @@ of the standard LaTeX environment.
   verse.
 - =:latexcode= :: It accepts any arbitrary LaTeX code that can be
   included within a LaTeX =verse= environment.
+- =:literal= :: With value t, all blank lines are preserved and
+  exported as =\vspace*{\baselineskip}=, including the blank lines
+  before or after contents.  Note that without the =:literal=
+  attribute, one or more blank lines between stanzas are exported as a
+  single blank line, and any blank lines before or after the content
+  are removed, which is more consistent with the syntax of the LaTeX
+  `verse' environment, and the one provided by the =verse= package.
+  If the =verse= package is loaded, the vertical spacing between all
+  stanzas can be controlled by the global length =\stanzaskip= (see
+  https://www.ctan.org/pkg/verse).
 
 A complete example with Shakespeare's first sonnet:
 

lisp/ox-latex.el

@@ -4116,6 +4116,7 @@ contextual information."
   (let* ((lin (org-export-read-attribute :attr_latex verse-block :lines))
          (latcode (org-export-read-attribute :attr_latex verse-block :latexcode))
          (cent (org-export-read-attribute :attr_latex verse-block :center))
+         (lit (org-export-read-attribute :attr_latex verse-block :literal))
          (attr (concat
 		(if cent "[\\versewidth]" "")
 		(if lin (format "\n\\poemlines{%s}" lin) "")
@@ -4128,20 +4129,37 @@ contextual information."
       verse-block
       ;; In a verse environment, add a line break to each newline
       ;; character and change each white space at beginning of a line
-      ;; into a space of 1 em.  Also change each blank line with
-      ;; a vertical space of 1 em.
+      ;; into a normal space, calculated with `\fontdimen2\font'.  One
+      ;; or more blank lines between lines are exported as a single
+      ;; blank line.  If the `:lines' attribute is used, the last
+      ;; verse of each stanza ends with the string `\\!', according to
+      ;; the syntax of the `verse' package. The separation between
+      ;; stanzas can be controlled with the length `\stanzaskip', of
+      ;; the aforementioned package.  If the `:literal' attribute is
+      ;; used, all blank lines are preserved and exported as
+      ;; `\vspace*{\baselineskip}', including the blank lines before
+      ;; or after CONTENTS.
       (format "%s\\begin{verse}%s\n%s\\end{verse}%s"
 	      vwidth
 	      attr
 	      (replace-regexp-in-string
-	       "^[ \t]+" (lambda (m) (format "\\hspace*{%dem}" (length m)))
+	       "^[ \t]+" (lambda (m) (format "\\hspace*{%d\\fontdimen2\\font}" (length m)))
 	       (replace-regexp-in-string
-                (concat "^[ \t]*" (regexp-quote org-latex-line-break-safe) "$")
-	        "\\vspace*{1em}"
+                (if (not lit)
+		    (rx-to-string
+                     `(seq (group ,org-latex-line-break-safe "\n")
+		           (1+ (group line-start (0+ space) ,org-latex-line-break-safe "\n"))))
+		  (concat "^[ \t]*" (regexp-quote org-latex-line-break-safe) "$"))
+	        (if (not lit)
+		    (if lin "\\\\!\n\n" "\n\n")
+		  "\\vspace*{\\baselineskip}")
 	        (replace-regexp-in-string
 	         "\\([ \t]*\\\\\\\\\\)?[ \t]*\n"
                  (concat org-latex-line-break-safe "\n")
-	         contents nil t)
+	         (if (not lit)
+		     (concat (org-trim contents t) "\n")
+		   contents)
+                 nil t)
                 nil t)
                nil t)
               linreset)

testing/lisp/test-ox-latex.el

@@ -61,11 +61,11 @@ lorem ipsum dolor
      (search-forward
       "\\begin{verse}
 lorem ipsum dolor\\\\[0pt]
+lorem ipsum dolor
+
 lorem ipsum dolor\\\\[0pt]
-\\vspace*{1em}
-lorem ipsum dolor\\\\[0pt]
-lorem ipsum dolor\\\\[0pt]
-\\vspace*{1em}
+lorem ipsum dolor
+
 lorem ipsum dolor\\\\[0pt]
 lorem ipsum dolor\\\\[0pt]
 \\end{verse}"))))