Update bibliography and citations in Vietnamese linguistics paper

- Add .DS_Store to gitignore
- Add Ngo (2021) reference to bibliography
- Fix citation format for Ngo (2021) in main text
- Add proper citations for Thompson, Cao, Hoàng, and Shimizu references

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Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -1 +1,2 @@
-__pycache__
+__pycache__
+.DS_Store

paper/main.bib

@@ -160,6 +160,14 @@
   sortkey = {17},
 }
 
+@book{ngo2021,
+  author    = {Ngo, Binh},
+  title     = {Vietnamese: An Essential Grammar},
+  publisher = {Routledge},
+  year      = {2021},
+  address   = {London, UK}
+}
+
 @book{nguyen1997,
   author = {{\fontencoding{T5}\selectfont Nguyễn}, {\fontencoding{T5}\selectfont Đình-Hoà}},
   title = {{\fontencoding{T5}\selectfont Vietnamese: Tiếng Việt không son phấn}},

paper/main.tex

@@ -49,7 +49,7 @@ The demand of learning Vietnamese as a second language in Vietnam and overseas i
 
 There is also a growing number of people speaking Vietnamese as a heritage language in the US, Cambodia, France, Australia and other countries. Vietnamese has been officially recognized as a minority language in the Czech Republic since 2013\footnote{\url{https://ec.europa.eu/migrant-integration/news/czech-republic-vietnamese-and-belarusians-recognized-ethnic-minorities_en}}. In Australia, Vietnamese occupies 82.7\% of the main languages spoken at home by Vietnam-born people there, according to the 2016 Census\footnote{\url{https://www.homeaffairs.gov.au/mca/files/2016-cis-vietnam.PDF}}.
 
-Responding to strong demand, numerous instructional textbooks have been published worldwide, including series by \citet{mai2007a}, \citet{vu2011}, \citet{doan2012,doan2014}, \citet{nguyen2019}, and more recently by \citet{phan2024vibes} and \citet{phung2025}. Additionally, there are descriptive guides to the Vietnamese language including \citet{nguyen1997} by John Benjamins,\footnote{\url{https://benjamins.com/catalog/loall.9}} and Ngo (2020) by Routledge.\footnote{\url{https://www.routledge.com/Vietnamese-An-Essential-Grammar/Ngo/p/book/9781138210707}}
+Responding to strong demand, numerous instructional textbooks have been published worldwide, including series by \citet{mai2007a}, \citet{vu2011}, \citet{doan2012,doan2014}, \citet{nguyen2019}, and more recently by \citet{phan2024vibes} and \citet{phung2025}. Additionally, there are descriptive guides to the Vietnamese language including \citet{nguyen1997} by John Benjamins,\footnote{\url{https://benjamins.com/catalog/loall.9}} and \cite{ngo2021} by Routledge.\footnote{\url{https://www.routledge.com/Vietnamese-An-Essential-Grammar/Ngo/p/book/9781138210707}}
 
 There is also a great leap in Vietnamese linguistic research over the past decade. There are three edited volumes devoted to Vietnamese: \cite{hole2013}, De Gruyter Mouton;\footnote{\url{https://www.degruyter.com/document/doi/10.1515/9783110289411/html?lang=en}} \cite{duffield2019}, John Benjamins;\footnote{\url{https://benjamins.com/catalog/slcs.211}} and \cite{phan2024springer}, Springer. Vietnamese has also been the focus of several special issues in Journal of East Asian Linguistics (2017),\footnote{\url{https://link.springer.com/journal/10831/volumes-and-issues/26-4}} Journal of the Southeast Asian Linguistics Society \cite{phan2022},\footnote{\url{https://evols.library.manoa.hawaii.edu/handle/10524/52500}} Taiwan Journal of Linguistics \cite{duffield2024,phan2025tjl}, and Languages \cite{phan2025lang}.
 
@@ -232,7 +232,7 @@ Falling & 312 (h{\fontencoding{T5}\selectfont ỏ}i) & \\
 
 \section{The phonemic system of Southern Vietnamese}
 
-According to the existing literature (Thompson 1959, {\fontencoding{T5}\selectfont Nguyễn} 1971, Thompson 1984-85, Cao 1988, {\fontencoding{T5}\selectfont Hoàng} 2004, Kondo 2016, Shimizu 2022), the main differences between Northern Vietnamese and Southern Vietnamese are as follows:
+According to the existing literature (\cite{thompson1959}, {\fontencoding{T5}\selectfont Nguyễn} 1971, \cite{thompson1984}, \cite{cao1988}, \cite{hoang2004}, Kondo 2016, \cite{shimizu2022}), the main differences between Northern Vietnamese and Southern Vietnamese are as follows:
 
 \begin{enumerate}
 \item[(i)] Initial consonants in Southern Vietnamese are more conservative than in Northern Vietnamese.