Creating expression tree in R

Question

The substitute function in R creates a language object in the form of a tree that one can parse. How can I create the tree from scratch using list or else to then g

Answer 1

> plus <- .Primitive("+")
> plus
function (e1, e2)  .Primitive("+")
> times=.Primitive("*")
> eval(call("plus", b, call("times",2, b)))
[1] 6
> eval(call("plus", a, call("times",2, b)))
[1] 5