A new transform op to represent that an attribute is to be chosen from a
set of alternatives and that this choice is made available as a
`!transform.param`. When a `selected` argument is provided, the op's
`apply()` semantics is that of just making this selected attribute
available as the result. When `selected` is not provided, `apply()`
complains that nothing has resolved the non-determinism that the op is
representing.
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