@Override
 protected void visit(RENode.CharacterClassExpr.Char expr) throws RuntimeException {
   solver = new Solver.Char(expr.getValue());
 }
 @Override
 protected void visit(RENode.CharacterClassExpr.Range expr) throws RuntimeException {
   RENode.CharacterClassExpr.Char from = expr.getFrom();
   RENode.CharacterClassExpr.Char to = expr.getTo();
   solver = new Solver.Range(from.getValue(), to.getValue());
 }