def resolve command_string
      if detected_command = @commands.detect{|cmd| cmd.matches? command_string }
        detected_command
      elsif parent_context
        parent_context.resolve(command_string)
      else
        raise "Command #{command_string} could not be resolved from the current context."
      end
    end
def resolve command_string
      if detected_command = @commands.detect{|cmd| cmd.matches? command_string }
        detected_command
      elsif parent_context
        parent_context.resolve(command_string)
      else
        raise "Command #{command_string} could not be resolved from the current context."
      end
    end
Look in current context and recursively through any available parent contexts to
 find definition of command. An Exception is raised if no implementation of command
 is found.