let rec remove_contact (a,b,c,d) agenda =
match agenda with
    [] -> []
    |(a1,b1,c1,d1)::l -> if string_lowercase a1 > string_lowercase a
                        then
                            remove_contact (a,b,c,d) l
                        else if string_lowercase a1 < string_lowercase a
                            then
                                (a1,b1,c1,d1)::l
                            else
                                l;;