It's in ast.go (https://github.com/golang/go/blob/master/src/go/ast/ast.go https://github.com/golang/go/blob/master/src/go/ast/ast.go). Look how Node, Expr etc.

It's in ast.go (https://github.com/golang/go/blob/master/src/go/ast/ast.go https://github.com/golang/go/blob/master/src/go/ast/ast.go). Look how Node, Expr etc. are interfaces, with concrete struct implementations. The interfaces don't do much, only provide position information.