Take a look at the second line in the `apply` definition[1]:

    self.eval @env[fn][2], Hash[*(@env[fn][1].zip args).flatten(1)]

It's triggered when the `fn` is not callable, i.e. when we pass in lambda:

    [:lambda, [:x], [:car, [:cdr, :x]]]

The code ignores the `:lambda` symbol (that would be `@env[fn][0]`).

It evals the third element of the list (the lambda body) in a new context that combines `@env` and the binding of lambda's args to the symbols in the lambda's definition -- that's what the `Hash` mumbo jumbo creates.

[1]: https://github.com/fogus/ulithp/blob/b02d5806ce3b2d3766311c2...