Class Dictionary:

RepetitionClass         = "~" <sandwiched> Sandwich(UNKNOWN1).

RepeatedPart            = 
  [ <nonempty> UNKNOWN2 ]
  "{" 
  <repeated> UNKNOWN3(ClassSpec)
  "}".

UNKNOWN4(S)             = 
  <first> List(UNKNOWN5) <inner> UNKNOWN6 <second> List(UNKNOWN7) .

ClassSpec               = ClassName.
UNKNOWN8             = <string> String.


Inputs:

A1 ~ {B}.
A2 ~ "x" {B} "y".
A3 ~ "x" {"r" B "s"} "y".
A4 ~ "x" B {"r" B "s"} "y".
A5 ~ "{" B {"," B} "}".