collections.Iterable considers pymbolic expressions iterable

[dokempf]

The implementation in https://github.com/inducer/pymbolic/blob/master/pymbolic/primitives.py#L609 causes collections.Iterable to consider all pymbolic expressions iterable (although they are explicitly not). I am not sure on the correct way to fix this, as I do not know the reason this error-raising implementation has been added. I am working around this for now, just wanted to let you know.

[inducer]

See #4 for some discussion of the rationale. Basically, without the definition of __iter__, numpy.array(pymbolic_obj) would lead to an infinite loop that.

On the other hand, defining __iter__ makes abc.Iterable believe that we're iterable. I'm not sure I see a solution that meets both of these requirements.