Do you really need the notation extension?

[Martin Thomson]

x(A..B) y(C..B-len(x)) indicates that x can be of any length between A and B, and y can be of any length between C and B provided that (len(x) + len(y)) does not exceed B.

This might be achieved as follows:

Outer {
  Inner (α..B),  <-- α = A + C
}
Inner {
  x(A..B),
  y(C..B),
}

[Martin Thomson]

I'll should also point out that α there is optional and you can also do:

Outer {
  Inner (..β),
}
Inner {
  x(A..B),
  y(C..D),
}

For any value of β, though presumably β is at least max(B, D), it could reasonably be any value up to B+D.

[Martin Duke]

closed by #207