Represents history of results of previously evaluated commands.
Available via __ variable, only if
IRB.conf[:EVAL_HISTORY] or
IRB::CurrentContext().eval_history is non-nil integer value
(by default it is nil).
Example (in `irb`):
# Initialize history IRB::CurrentContext().eval_history = 10 # => 10 # Perform some commands... 1 + 2 # => 3 puts 'x' # x # => nil raise RuntimeError # ...error raised # Inspect history (format is "<item number> <evaluated value>": __ # => 1 10 # 2 3 # 3 nil __[1] # => 10