{AST} is a library for manipulating abstract syntax trees.

It 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} for
additional recommendations and design patterns.