about calcite core design

[xuxuwangzi888]

dear,
      I have a question,why the parser and operator Tree are coupled，like this:

SqlNode(SqlParserPos pos) {

  Util.pre(pos != null, "pos != null");
  this.pos = pos;
}


  Because we pass the variable pos，it lead to the couple。

I develped a parser use antlr，and hope the parse and symantec tree is loose 
coupled。
    
   I guess calcite coupled design is for performance, is it ?   

   Many thanks!

gongbo.yxh