mutable x -- returns true or false, depending on whether x is mutable.
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.