'isimmutable' and 'ImmutableNester'

[Frank-Rene Schäfer]

A tuple is immutable but it may contain mutable objects. In larger
hierarchies of objects it may become less obvious whether down
the lines, there is some mutable object somewhere in the data tree.

One can define a recursive function to check for immutability
manually. However first, it may not be as efficient as if it was
built-in. Second, the existence of a built-in function 'isimmutable'
puts the concept of immutability some more into the spotlight.

You might indeed implement some personal 'policy for copy/deepcopy'.
But, how can you prevent the insertion of an object into the data
tree which does not follow your copy/deepcopy convention? As soon
as you allow members of type 'tuple' you must either check recursively
or only allow ints and strings as tuple members.