About producing the 2-variables expression

 In previous part, I suggested a solution for single-variable expression. In this case, the solution for producing a new expression is quite clear and is clarified with 3 different examples. However, in case we give an expression with 2-varibles with different levels. How can we process this type of expression without re-implementing a new function? That's why we need to consider a new solution to re-use the current solution with single-variable.

 This time, the select-expression will be parsed into many different parts. And then we will have 2-nested select-expressions. And what is the next step with the nested-expression?