Just opened this one in the context of a slack discussion. Its not intended to be merged or reviewed and will be closed once we end the discussion.
This will allow writing (in main flow, loops and retries)
Either unnamed single tasks (this assumes singles tasks can be univocally referenced through json pointers, which I believe is a valid one)

do:
   set:

on named multiple tasks (multiple tasks needs to be named to be reference univocally)

do: 
  task1: 
    set:
do:
  task2: 
    call: http

This will prevent writing

do:
  set: 
    set: 

which is valid now for main flow (it defines a task named set which invoke set tasks)