code: public BtrpOperand expand() {
          String head = getChild(0).getText().substring(0, getChild(0).getText().length() - 1);
          String tail = getChild(getChildCount() - 1).getText().substring(1);
          BtrpSet res = new BtrpSet(1, BtrpOperand.Type.STRING);
  
          for (int i = 1; i < getChildCount() - 1; i++) {
              BtrpOperand op = getChild(i).go(this);
              if (op == IgnorableOperand.getInstance()) {
                  return op;
              }
              BtrpSet s = (BtrpSet) op;
              for (BtrpOperand o : s.getValues()) {
                  //Compose
                  res.getValues().add(new BtrpString(head + o.toString() + tail));
              }
          }
          return res;
      }
docs: Expand the enumeration.
  Variables are not evaluated nor checked
  
  @return a set of string or an error