Name:
  Filter

Intent:
  - apply composable transformations to a stream of elements
    STREAM, not array or list

Structure:
  - transformations are chained line in a pipeline
  - all the tranformations have the same interface
  - there are two key methods
     - connect to the previous module in the chain
         statically is constructor, dynamically is setter
     - pull an element from the previous module
  - there is an initial element in the chain (source)

Notes:
  - there is symmetric (specular) version
      - previous -> next
      - pulling -> pushing
      - source -> sink