AST in a more general concept see https://en.wikipedia.org/wiki/Abstract_syntax_tree, it basically turn your text into a tree where f(a) + f(b) is `call(plus, call(f, a), call(f,b))`. Using https://github.com/alexmojaki/stack_data we can say "the error occurred in `a`, while trying to call `f`, while calling `plus`, get the range in the original text and make them yellow.
