Multiple repeat operators

The following snippet
```
A <- B C?
B <- "B"
C <- [a-z]+
```
produces the error
```
pe._errors.GrammarError: multiple repeat operators
```
upon compiling it.

It seems like pe thinks that the `?` and the `+` operator follow each other immediately and ignores the rule indirection. However, I think a grammar like the above seems perfectly normal.
Therefore, this seems like a bug to me.