Typeset a single `grammar` block with multiple productions

[sgraf812]

I'd like to typeset several productions of a grammar at once, such that it displays roughly as

term ::= ...
  | term + term
  | term * term
  | term / term

I tried the subjectively intuitive thing, writing one grammar block and separating each production by a newline:

```grammar
$_:term + $_:term
$_:term * $_:term
$_:term / $_:term
```

But that displays as

term ::= ...
  | term + term
    term * term
    term / term

and actually parses everything as one big production, it seems.