== Immutable data ==
Immutability means that there is '''no over-writes''' of data in principle. A ''new'' copy (version) of the data structure is created instead.
This has a huge implication, namely it guarantees that once defined a value stays the same, no matter what. This mimics a fundamental notion from math - {{{a = 1}}} is an assertion of truth, while {{{a = a + 1}}} is nonsense.
This is also called Referential Transparency.