"Prettier" design pattern than nested pattern matching? OCaml

I'm doing a uni course on OCaml and need to parse an s-expr into it's proper AST representation, using the Containers library

I need to extend the parser to include boolean values but I can't find an elegant way to pattern match the string over just nesting more pattern matches.

So I wonder if there's a prettier way to do it.

So far if the parser detects an `Atomic string "s" it "opens" a nested match case to see if it's an int or a variable name.

...| `Atomic s ->  begin    match int_of_string_opt s with    | Some n -> Num n    | None ->      begin        match bool_of_string_opt s with        | Some b -> Bool b        | None -> Var s      end  end...

This solution works but just by looking I can tell it could be designed better but I'm not familiar enough with OCaml to know how to make it better.