We use `nullKind` for the `group` parser combinator.
When pattern matching `nullKind` nodes, we check their arities.
So, error recovery often fails for parsers that use the `group`
combinator.
For example, we have the parser
```
def whereDecls := leading_parser "where " >> many1Indent (group (letRecDecl >> optional ";"))
```
If there is syntax error at `letRecDecl`, the node corresponding to
```
group (letRecDecl >> optional ";")
```
will contain only one child, and the pattern matching at
```
def expandWhereDecls (whereDecls : Syntax) (body : Syntax) : MacroM Syntax :=
match whereDecls with
| `(whereDecls|where $[$decls:letRecDecl $[;]?]*) => `(let rec $decls:letRecDecl,*; $body)
| _ => Macro.throwUnsupported
```
fails, and we can't elaborate the partial syntax tree for
`letRecDecl`, and auto-completion will not work there.
We address this issue by using a new kind for the `group` combinator.
The idea is to pattern match `group` as we pattern match `node`s with
proper syntax node kinds. This change is consistent with the way we
use `group` where it mainly a convenience for saving us the trouble of
defining a new parser definition that is used only once.
This kind of broken command does not have a position information, and
was producing a panic message in the server because it makes the
reasonable assumption that every command returned by `parserCommand`
has a position.
We should never assume a `Syntax` node has a specific number of
children because the parser error recovery may produce partial
abstract syntax trees. We should use `stx[i]` instead because
`Syntax.getOp` returns `Syntax.missing` when `i` is out of bounds.
I am hitting many error recovery problems. This is an attempt to make
auto-completion more robust.
For example, we currently can't auto complete inside of parentheses.
The syntax tree for the syntactically incorrect tern `(s. , 0)` is
```
(Term.paren "(" [(Term.proj `s ".")])
```
The elaborator for `paren` fails to match this partial syntax tree and
returns an error. I considered adding code to the `else` branch of
this elaborator and handle partial trees there. However, we would
still be losing information for the other elements of the tuple.
Example: no hover information for them.
The helper parser makes sure we don't lose information.
