docs: private static String getFreeVariable(Expression2 expression, int index, Scope scope) {
      Expression2 sub = expression.getSubexpression(index);
      if (sub.isConstant()) {
        String constant = sub.getConstant();
        if (!constant.equals(LAMBDA) && !scope.isBound(constant)) {
          return sub.getConstant();
        } 
      }
      return null;
    }