Without a key parameter, neither the main block nor the else block is rendered.

Literal values work in both keys and values.

Can't Walk Theory
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Assuming a missing value = cantwalk and a non-existent key = cantwalk then their equality makes sense.

The null tests have proven that absent parameters and missing values do not equal null and therefore it is not just making all falsey values equal.

Non-Scalar Values
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For non-scalar values, it seems that dust does mark two values as true if they have the same path, but otherwise they are not equal.