instance_exec

        # BasicObject.instance_exec

(from ruby core)
---
    obj.instance_exec(arg...) {|var...| block }                       -> obj

---

Executes the given block within the context of the receiver (*obj*). In
order to set the context, the variable `self` is set to *obj* while the
code is executing, giving the code access to *obj*'s instance variables.
 Arguments are passed as block parameters.

    class KlassWithSecret
      def initialize
        @secret = 99
      end
    end
    k = KlassWithSecret.new
    k.instance_exec(5) {|x| @secret+x }   #=> 104