This is a Ruby tree! It shows every object from the Ruby Programming Language in a tree format.

cause 

        # Exception.cause

(from ruby core)
---
    exception.cause   -> an_exception or nil

---

Returns the previous exception ($!) at the time this exception was
raised. This is useful for wrapping exceptions and retaining the
original exception information.

This is MURDOC! A Ruby documentation browser inspired by Smalltalk-80. It allows you to learn about Ruby by browsing through its class hierarchies, and see any of its methods.