array.c.doc

Comparison---Returns an integer (-1, 0,
or +1) if this array is less than, equal to, or greater than
other_ary.  Each object in each array is compared
(using <=>). If any value isn't
equal, then that inequality is the return value. If all the
values found are equal, then the return is based on a
comparison of the array lengths.  Thus, two arrays are
``equal'' according to Array#<=> if and only if they have
the same length and the value of each element is equal to the
value of the corresponding element in the other array.

   [ "a", "a", "c" ]    <=> [ "a", "b", "c" ]   #=> -1
   [ 1, 2, 3, 4, 5, 6 ] <=> [ 1, 2 ]            #=> +1