2018-03-21 09:24:00

1 files changed, 31 insertions, 4 deletions

diff --git a/tex/context/base/mkiv/spac-hor.mkiv b/tex/context/base/mkiv/spac-hor.mkiv
index 176b52e1f..ce747a202 100644
--- a/tex/context/base/mkiv/spac-hor.mkiv
+++ b/tex/context/base/mkiv/spac-hor.mkiv
@@ -680,6 +680,7 @@
                                    \global        \s_spac_narrower_middle \zeropoint
                                    \global        \s_spac_narrower_right  \zeropoint\relax}
 \installnarrowermethod \v!none    {}
+\installnarrowermethod \v!reverse {} % never seen
 
 \unexpanded\def\spac_narrower_start#1%
   {\begingroup
@@ -693,19 +694,45 @@
      \spac_narrower_start_apply{\narrowerparameter\v!default}%
    \fi}
 
+\newskip\s_spac_narrower_left_last
+\newskip\s_spac_narrower_right_last
+\newconditional\s_spac_narrower_last_swap
+
 \def\spac_narrower_start_apply#1%
   {\narrowerparameter\c!before
    \global\s_spac_narrower_left  \zeropoint
    \global\s_spac_narrower_right \zeropoint
    \global\s_spac_narrower_middle\zeropoint
-   \normalexpanded{\processcommalistwithparameters[#1]}\spac_narrower_initialize
-   \advance\leftskip \dimexpr\s_spac_narrower_left +\s_spac_narrower_middle\relax
-   \advance\rightskip\dimexpr\s_spac_narrower_right+\s_spac_narrower_middle\relax
+   \edef\askednarrower{#1}
+   \ifx\askednarrower\v!reverse
+     \ifconditional\s_spac_narrower_last_swap
+       \leftskip \s_spac_narrower_right_last
+       \rightskip\s_spac_narrower_left_last
+       \setfalse\s_spac_narrower_last_swap
+     \else
+       \leftskip \s_spac_narrower_left_last
+       \rightskip\s_spac_narrower_right_last
+       \settrue\s_spac_narrower_last_swap
+     \fi
+   \else
+     \normalexpanded{\processcommalistwithparameters[\askednarrower]}\spac_narrower_initialize
+     \advance\leftskip \dimexpr\s_spac_narrower_left +\s_spac_narrower_middle\relax
+     \advance\rightskip\dimexpr\s_spac_narrower_right+\s_spac_narrower_middle\relax
+   \fi
    \seteffectivehsize}
 
 \unexpanded\def\spac_narrower_stop
   {\narrowerparameter\c!after
-   \endgroup}
+   \normalexpanded{%
+      \endgroup
+      \s_spac_narrower_left_last \the\leftskip \relax
+      \s_spac_narrower_right_last\the\rightskip\relax
+      \ifconditional\s_spac_narrower_last_swap
+        \setfalse\s_spac_narrower_last_swap
+      \else
+        \settrue\s_spac_narrower_last_swap
+      \fi
+   }}
 
 \unexpanded\def\startnarrower
   {\dosingleempty\spac_narrower_start_basic}