@Kha Note that I had to write the weird pattern
```
match_syntax stx with
| `(notation:$prec $items* => $rhs) => expandNotationAux stx prec items rhs
| `(notation $noprec* $items* => $rhs) => expandNotationAux stx none items rhs
| _ => Macro.throwUnsupported
```
with the weird `$noprec*` to match the case where the optional
precedence is not provided. I realized this is not a bug, but
I guess most users will be puzzled by this behavior. If we had a kind
for `notationItem`, I would be able to write
```
`(notation $items:notationItems* => $rhs)
```
Before this commit, if we execut `longestMatch [p1, p2]` in a
trailing parser where both `p1` and `p2` succeed, then an incorrect
syntax tree is generated by `p2`. The issue is that before executing
`p2`, the trailing stack is of the form
`#[..., left, syntax-by-p1]`
Then, the trailing parser `p2` incorrectly assumes that `syntax-by-p1`
was the "left" syntax node.
This commit fix this issue by storing the `left` node at the
`ParserContext` at `trailingLoop`
@Kha This bug was introduced when we unified leading and trailing
parsers. I think this is the simplest solution.
We can observe the bug before this commit by using
```
set_option syntaxMaxDepth 1000
set_option trace.Elab true
```
at `choiceMacroRules.lean`.
It is funny the correct result was produced even with the bug.
It was working because the `macro_rules` was matching the
buggy syntax with the nested `syntax-by-p1` there :)
@Kha I implemented the solution 1 I described at Zulip.
I also tried to document the issue, and made sure test
`choiceMacroRules` fail if the bug is reintroduced.
@Kha Could you please double check these modifications.
I added a no-op for `checkRbpLt`. It is used at the `Sort` and `Type`
parsers.
As I described in previous commits, the `checkRBPGreater` comment and
implementation were misleading. It was actually succeeding when the
rbp was less than or equal to the given parameter. So, it was renamed
to `checkRbpLe`. So, is the depArrow parenthesizer ok? I did not check.
I updated the PPRoundtrip.lean.expected.out to make sure the tests
succeed, but we should revise it if there is a problem with the
modifications at Parenthesizer.lean
