If x is a hash table, list, or database, then it's mutable if its contents can be destructively altered.
If x is a symbol, then it's mutable if a value can be assigned to it. (See protect.)
If x is anything else, then it isn't mutable.
The contents of a mutable hash table do not participate in strong comparison with === or in hashing.
See also MutableList and MutableHashTable.
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