Parentheses in grammar

[LinkTed]

Hi guys,
First, thanks for this awesome library.

I do not know if this is the right place to ask, but I tried to implement the following parser:

1. The text has lines that are separated by "\n". Like this a = { b ~ c ~ d ~ "\n" }
2. This means that no "\n" has to appear between the rules b, c and d.
3. But there is an expectation that you can have multiline support by using parentheses. So this means that if there are parentheses, the new line as a separation should be ignored. Like a = "(" ~ "\n"? ~ b ~ "\n"? ~ ")"~ c ~ d ~ "\n"
4. The parentheses can appear between (b and c) or (b and d) or (c and d).

Something like this:

a = { b ~ c ~ d  ~ "\n" |  
      "(" ~ "\n"? ~ b ~ "\n"? ~  ")"~ c ~ d  ~ "\n"  |
      "(" ~ "\n"? ~ b ~ "\n"?  ~  c ~ "\n"? ~  ")" ~ c ~ d  ~ "\n" |  etc. }
b = { "b" }
c = { "c" }
d = { "d" }

I tried to use the stack, but I did not figure it out. Maybe you guys have a nice solution, where I do not have to write any combination of the rules in parentheses.