expression cannot be chained in letting clause

load gamesim-sample

run this query:

 select level from `gamesim-sample` where level is not missing group by level letting b=1,c=b+1

give back this error:

"Expression must be a group key or aggregate: (`b` + 1)"

We should be able to chain expressions in letting clause independent of group keys

same thing happens if b is not an expression but a field:

select level from `gamesim-sample` where level is not missing group by level letting b=level,c=b+1

gives same error