def irb_def(symbol, method = nil, &block)
      irb_rc {
        IRB::ExtendCommandBundle.class_eval {
          define_method(symbol, method || block)
        }
      }
    end
def irb_def(symbol, method = nil, &block)
      irb_rc {
        IRB::ExtendCommandBundle.class_eval {
          define_method(symbol, method || block)
        }
      }
    end
call-seq:
   irb_def(symbol) { ... }
   irb_def(symbol, method)

 Define a method for use inside the IRb session.