Thanks! Fixed now

Fixed, only generateTestCase and deepCopy are exposed now.

I'm not sure as to how I should do this with an IIFE - I need to access for example generateTestCase multiple times within the test script.
Could you give me an example as to how I could solve this with iffys?

Should be fixed now

Reopened: (Initially posten by Voile)

If user has defined isInFix themselves they'll get SyntaxError: Identifier 'isInfix' has already been declared.
Should use an IIFE to isolate your preloaded helper functions so they don't pollute the user code scope.

Thank you very much for pointing this stuff out!
Fixed both of the last two bullets, but I can't reproduce the first one just yet

Should be fixed now

It allows for more solutions to the kata. One solution may contain more expressions, but turn out to be the same answer.

You were right, I've modified my solution based on yours.
Btw:

  { op: '~', inner: 't0', out: 't0' },
                     ~~ Should be 'xy'

Right. Could you show me this example solution which only uses two temporary variables?

Fixed, thanks for pointing it out!

But it should not matter, see response below

Copied AST for the user function call

Uh-oh - will look into that!

Could you show me this example solution which only uses two expressions?

I'm thinking it's probably not clear from the description. You're not supposed to fold constants in this kata