Add {gather} support

luamml-patches-amsmath.sty

@@ -3,6 +3,12 @@
 
 \lua_now:n { require'luamml-amsmath' }
 
+% For all of these changes, the redefinitions appear huge.
+% But they are almost identical to the original and only
+% add luamml commands in appropriate places, so they would
+% mostly disappear if there were enough hooks in amsmath.
+
+% aligned and friends
 \cs_set:Npn \start@aligned #1#2 {
   \RIfM@
   \else
@@ -66,6 +72,66 @@
   \__luamml_amsmath_finalize_inner_table:
 }
 
+% gather
+\cs_set:Npn \gather@ #1 {
+  \ingather@true
+  \let \split \insplit@
+  \let \tag \tag@in@align
+  \let \label \label@in@display
+  \chardef \dspbrk@context \z@
+  \intertext@ \displ@y@ \Let@
+  \let \math@cr@@@ \math@cr@@@gather
+  \gmeasure@ {#1}
+  \global \shifttag@false
+  \tabskip \z@skip
+  \global \row@ \@ne
+  \halign to \displaywidth \bgroup
+      \strut@
+      \setboxz@h {
+        $
+          \m@th
+          \displaystyle
+          {##}
+          \luamml_flag_save:Nn \displaystyle {mtd}
+        $
+      }
+      \__luamml_amsmath_add_box_to_row:
+      \calc@shift@gather
+      \set@gather@field
+      \tabskip\@centering
+    &
+      \setboxz@h {
+        \strut@
+        {##}
+      }
+      \place@tag@gather
+      \__luamml_amsmath_set_tag:
+      \tabskip \iftagsleft@
+        \gdisplaywidth@
+      \else
+        \z@skip
+      \span \fi
+      \crcr
+      #1
+}
+\cs_new_eq:NN \__luamml_amsmath_original_gmeasure:n \gmeasure@
+\cs_set:Npn \gmeasure@ #1 {
+  \exp_last_unbraced:Nno
+    \use_ii_i:nn
+    { \luamml_flag_ignore: }
+    { \__luamml_amsmath_original_gmeasure:n {#1} }
+}
+
+\cs_set:Npn \endgather {
+      \math@cr
+      \black@ \totwidth@
+      \__luamml_amsmath_finalize_table:
+  \egroup
+  $$
+  \ignorespacesafterend
+}
+
+% align and friends
 \cs_set:Npn \align@preamble {
   &
     \hfil