@Kha I had to do this because of the `ident` vs `Term.id` recurrent
issue. `match_syntax` fails if a `Term.id` is used at `Term.letIdDecl`
where an `ident` is expected.
We should try to remove `Term.id` in the future.
@Kha the patterns at `Binders.lean` for let-expressions are not matched correctly by
`match_syntax`. The problem is that we "reuse" kinds here:
```lean
def letIdDecl : Parser := nodeWithAntiquot "letDecl" `Lean.Parser.Term.letDecl $ try (letIdLhs >> " := ") >> termParser
def letPatDecl : Parser := node `Lean.Parser.Term.letDecl $ try (termParser >> pushNone >> optType >> " := ") >> termParser
def letEqnsDecl : Parser := node `Lean.Parser.Term.letDecl $ letIdLhs >> matchAlts false
def letDecl := letIdDecl <|> letPatDecl <|> letEqnsDecl
```
This is a bad hack for implementing the `where` macro. I will remove it, and rewrite the code above as
```lean
def letIdDecl := parser! try (letIdLhs >> " := ") >> termParser
def letPatDecl := parser! try (termParser >> pushNone >> optType >> " := ") >> termParser
def letEqnsDecl := parser! letIdLhs >> matchAlts false
def letDecl := parser! letIdDecl <|> letPatDecl <|> letEqnsDecl
```
Remark: we need the `letDecl` kind to be able to implement the `where`
macro.
I will do it tomorrow because it is a staging nightmare.
TODO: the structure instance `..` is not being handled correctly in
patterns. We must create new pattern variables for the missing fields.
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
We go back to the original approach where we pattern matching
alternative variables as `FVar`s.
We fix the original problem we had by implementing a simple
unification procedure for alternative `FVar`s.
@Kha Please try to avoid importing `Lean.Meta`, it is huge, and any
change there was forcing a bunch of files to be recompiled since
`Parser.Extension` depends on `Parenthesizer`
