ERROR: /path/ext.bzl:7:1: Variable b is read only
You will get a similar error if you try to redefine a function (function
overloading is not supported), for example:
Error: Variable x is read only
A global variable cannot be reassigned. It will always point to the same object.
However, its content might change, if the value is mutable (for example, the
content of a list). Local variables don’t have this restriction.
⌘I