Modified r_list_copy to r_proper_part

R_proper_part takes the proper part of a dotted-list, for example:

    r_proper_part ('(1 . 2)) == '(1)
    r_proper_part ('(1 2 . 3)) == '(1 2)
    r_proper_part ('(1 2 3)) == '(1 2 3)