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?