--- pretty_print_instance_variables
#    returns a sorted array of instance variable names.

    Ȥ줿󥹥ѿ֤̾ޤ

#    This method should return an array of names of instance variables as symbols or strings as:

    Υ᥽åɤϥ󥹥ѿ̾򥷥ܥ뤫ʸȤǤ˻
    ֤ʤФʤޤ
    [:@a, :@b].

