Lean: `varswap` test fails

[protoben]

The varswap Lean test fails with the following message:

Type error:
varswap.sail:11.8-9:
11 |    (x, y) = (y, x);
   |        ^
   | Cannot modify immutable let-bound constant or enumeration constructor y

Failed: custom_flow.sail timeout 90s lake exe run > expected 2> err_status

[javra]

This looks similar to what you were fixing recently, @Alasdair, right? That this should be expanded to a series of let bindings?

[ineol]

When I tried today it compiled but the output sail is wrong:

$[complete]
function main () = let x = "Hello" in
let y = "World" in
let _ : unit =
  let (tup__0, tup__1) = ((y, x)) : (string, string) in
  let x : string = tup__0 in
  let y : string = tup__1 in ()
in
internal_plet _ : unit = print_endline(y) in print_endline(x)

the internal lets are lost