Declaration syntax:

<left> := <right>

Mutable assignment syntax:

<left> ?= <right>

Where ?= can be =, +=, *= etc.

The following rules are used:

• := does a shallow clone of right value, if it is a reference
• = does a shallow clone of right value
• := ignores the type of the assigned item
• ?= checks the type of assigned item
• ?= does a shallow clone (copy-on-write) of left value before assigning, if it is a reference

The rules are designed for:

• Make numbers and booleans feel natural without the runtime distinguishing between copy and non-copy variables
• Allow sharing of objects, but with the ability to replace it with a non-shared object
• Copy-on-write behavior, where no simple assignment mutates an object elsewhere
• Avoid making variables entangled in surprising ways
• Prevent infinite cycles of references