module Byebug::ParseFunctions

def show_onoff(bool)

be true, false or nil.
Return 'on' or 'off' for supplied parameter. The parameter should 
def show_onoff(bool)
  if not [TrueClass, FalseClass, NilClass].member?(bool.class)
    return "??"
  end
  return bool ? 'on' : 'off'
end