docs: Update tabstop-generation README (#1272) (#1298)

- Added multi-line placeholder creator

doc/examples/tabstop-generation/README.md

@@ -48,3 +48,59 @@ endsnippet
 
 Snippet is declared via regular expression and will expand to any required
 number of fields in row.
+
+Now, for a more robust tabstop generation, to create placeholders within
+multiple lines:
+
+```
+global !p
+def create_matrix_placeholders(snip):
+	# Create anonymous snippet body
+	anon_snippet_body = ""
+
+	# Get start and end line number of expanded snippet
+	start = snip.snippet_start[0]
+	end = snip.snippet_end[0]
+
+  # Append current line into anonymous snippet
+	for i in range(start, end + 1):
+		anon_snippet_body += snip.buffer[i]
+		anon_snippet_body += "" if i == end else "\n"
+
+	# Delete expanded snippet line till second to last line
+	for i in range(start, end):
+		del snip.buffer[start]
+
+	# Empty last expanded snippet line while preserving the line
+	snip.buffer[start] = ''
+
+	# Expand anonymous snippet
+	snip.expand_anon(anon_snippet_body)
+
+def create_matrix(cols, rows, sep, start, end):
+	res = ""
+	placeholder = 1
+	for _ in range(0, int(rows)):
+		res += start + f"${placeholder} "
+		placeholder += 1
+		for _ in range(0, int(cols) - 1):
+			res += sep + f" ${placeholder} "
+			placeholder += 1
+		res += end
+	return res[:-1]
+endglobal
+
+post_jump "create_matrix_placeholders(snip)"
+snippet 'arr(\d+),(\d+)' "LaTeX array" br
+\begin{array}{`!p
+orient = ""
+for _ in range(0, int(match.group(1))): orient += "l"
+snip.rv = orient`}
+`!p
+snip.rv = create_matrix(match.group(1), match.group(2), "&", "\t", "\\\\\\\\\n")
+`$0
+\end{array}
+endsnippet
+```
+
+![gif](demo1.gif)