a_in_1: all different words
a_in_2: one word that does not fit by one byte
a_in_3: several words with some successive duplicates
a_in_4: one very long word

c_in_1: in lab8a.c, for MAX_SIZE 10 will not append anything.
d_in_1: in lab8a.c, for MAX_SIZE 40 will append one full word.

b_in_1: two twin words of odd and even length
b_in_2: mix of twin and non-twin
b_in_3: non-twin if scanf's format string is correct
