E$7\mathrm{.}21$ Add a method

public Sequence merge$($Sequence other$)$

to the Sequence class of Exercise E$7\mathrm{.}20$ that merges two sequences, alternating ele$-$

ments from both sequences. If one sequence is shorter than the other, then alternate

as long as you can and then append the remaining elements from the longer

sequence. For example, if a is

$1,4,9,16$

and $b$ is

$9,7,4,9,11$

then a$.$merge$($b$)$ returns the sequence

without modifying a or b$.$

$1,9,4,7,9,4,16,9,11$