ruby-ast - 2.4.1-1 main

ast embraces immutability; each AST node is inherently frozen at
creation, and updating a child node requires recreating that node
and its every parent, recursively.
.
This is a design choice. It does create some pressure on
garbage collector, but completely eliminates all concurrency
and aliasing problems.
.
See also AST::Node, AST::Processor::Mixin and AST::Sexp classes for
additional recommendations and design patterns.