Algorithm

   It was 5 days from the time I met Prof. There is not any progress since then. What's the main reason? At first, I just gave an example about the f-select and g-extend expressions in very complicated example. Then I just focus on solving that problem and forget about the compositional expression.
   After thinking a lot, I recognize that the main problems I need to concentrate on the very simple type. For example, I need to solve this first:
      f: select $db
      g: extend epsilon->$v with {} in $db
   The 3 main cases that I must implement so that I could make compositional functions are:
      {}              : create a single node graph.
      {a:G}        : an edge pointing to a graph.
      G1 U G2   : graph union.
  That's what I need to consider. About the {}: in pattern, its type is constant.
  I have to modify and fix the function that I implemented with other approaches.