There is an easy way to check for yourself that the dict build-in does
not have a hash method:
In [1]: d = {1:1}
In [2]: hash(d)
TypeError: unhashable type: 'dict'
Hashes are not defined on mutable objects because the hash of an
object should not change. Thanks to this quality hashes can be used
for cashing.
Cashing and hashes are an important part of the core, this is why for
the moment if you implement something in sympy it should be immutable.
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