Implement var-args for macros

[grayswandyr]

Macros allow to implement an unchanged macro as:

let unchanged[x] { (x) = (x)' }

But, in every event, this macro should be called for every unchanged relation (or part of, e.g. with <:), as in:

var sig S { var r1, r2 : set S }
...
pred ev[...] {
  ...
  unchanged[S]
  unchanged[r1]
  unchanged[x <: r2]
  ...
}

Can we implement a single call to unchanged? Almost, using meta... The following is possible:

let allUnchanged[X] { all x: X | x.value = (x.value)' }
pred ev[...] {
  ...
  allUnchanged[S$ + S$r1] // won't work with `x <: r2`
  unchanged[x <: r2]
  ...
}

The allUnchanged macro is nice but (1) it must be called with ugly meta-arguments (incl. fully-qualified meta-names like S$r1) and (2) it only works for fully-unchanged stuff (it won't work on x <: r2.

I am here proposing that we add comma-delimited var-args to macros as well as an args$ meta-sig, local to its enclosing macro, so that something like this could be written:

let allUnchanged[X] { all x : args$ | x = (x)' }
pred ev[...] {
  ...
  allUnchanged[S, r1, (x <: r2)]
  ...
}