F# Indexer

(In learning from "Programming F# 3.0, 2nd Edition")

Like ruby, F# can define indexer as defining Item property.

defining

member this.Item (idx : int) =
  ...

accessing

xxx.[42]

And also like ruby, indexer can take one more arguments.

defining

member this.Item (prefix : string , idx : int) =
  ...

accessing

xxx.["Answer", 42]