> - remove the checks from Perm, and make a *function* PermS (the name is
> deliberately chosen to be as close as Perm) that performs the checks and
> then constructs a permutation object using Perm (the name comes from
> Permutation Safe).

I am in favor of this. `Basic` subclasses should exist for
mathematical concepts, not for user convenience (especially when we
take the need for reconstruction of these classes and their expression
trees into account). A helper function is a much better solution than
some magic done in the `__new__` method.

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