unable to infer monad type at nested monad use? "do perror..."

[bollu]

partial def parse_if (u: Unit) : P Stmt := do
 let p_is_fi_or_elif_or_else : P Bool :=
     por (ppeek_keyword? "fi") (por (ppeek_keyword? "elif") (ppeek_keyword? "else"))

 let rec pelse (u: Unit)  : P  ((List (Expr × List Stmt)) × Option (List Stmt)) := do
   match (<- ppeek_keyword) with
      -- | some "elif" => do
      --        let cond <- parse_expr u
      --        pconsume_keyword "then"
      --        let body <- parse_stmts p_is_fi_or_elif_or_else u
      --        let (elifs, else_) <- pelse u
      --        return ((cond,body)::elifs, else_)
      -- | some "else" =>  do
      --     pconsume_keyword "else"
      --     let stmts <- parse_stmts (ppeek_keyword? "fi") u
      --     pconsume_keyword "fi"
      --     return ([], Option.some stmts)
      -- | some "fi" => 
      --   pconsume_keyword "fi"
      --   return ([], Option.none)
      | _ => perror "expected elif/else/fi at end of if" <- CANNOT INFER