multiline not recognised [closed]

Question

I am trying to use multiline with Xelatex in Texshop but this is what happens when I do. This is a minimal document:

>\documentclass{amsart}  
\usepackage{amsmath}  
\begin{document}  
\begin{multiline}  
    a+b+c+d+e+f=g\\  
    l+m=n  
\end{multiline}    
\end{document}    

And the console output is:

This is XeTeX, Version 3.14159265-2.6-0.99996 (TeX Live 2016) (preloaded format=xelatex)
 restricted \write18 enabled.
entering extended mode
(./mwe.tex
LaTeX2e <2016/03/31>
Babel <3.9r> and hyphenation patterns for 83 language(s) loaded.
(/usr/local/texlive/2016/texmf-dist/tex/latex/amscls/amsart.cls
Document Class: amsart 2015/03/04 v2.20.2
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsmath.sty
For additional information on amsmath, use the `?' option.
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amstext.sty
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsgen.sty))
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsbsy.sty)
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsmath/amsopn.sty))
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/umsa.fd)
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/amsfonts.sty))
(./mwe.aux) (/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/umsa.fd)
(/usr/local/texlive/2016/texmf-dist/tex/latex/amsfonts/umsb.fd)
./mwe.tex:5: LaTeX Error: Environment multiline undefined.  
See the LaTeX manual or LaTeX Companion for explanation.  
Type  H <return>  for immediate help.  
 ...                                                
 l.5  \begin{multiline}

I suspect I am doing something silly but I would be very glad to be told what. Thanks