LaTeX reader: Support `\let`.

Also, fix regular macros so they're expanded at the
point of use, and NOT also the point of definition.
`\let` macros, by contrast, are expanded at the
point of definition.  Added an `ExpansionPoint`
field to `Macro` to track this difference.

1 files changed, 23 insertions, 0 deletions

diff --git a/test/command/macros.md b/test/command/macros.md
index 055c86d25..49a648c79 100644
--- a/test/command/macros.md
+++ b/test/command/macros.md
@@ -15,3 +15,26 @@ $\my+\my$
 \newcommand{\my}{\phi}
 $\my+\my$
 ```
+
+`\let` macros should be expanded at point of
+definition, while `\newcommand` macros should be
+expanded at point of use:
+
+```
+% pandoc -f latex -t latex
+\let\a\b
+\newcommand{\b}{\emph{ouk}}
+\a
+^D
+\b
+```
+
+```
+% pandoc -f latex -t latex
+\newcommand{\a}{\b}
+\newcommand{\b}{\emph{ouk}}
+\a
+^D
+\emph{ouk}
+```
+