Students do not have the necessary knowledge at this point to understand
what's happening with the iterator combinators. This topic is covered
well by the dedicated exercises about iterators later.
closes#2102
The original excercise instructs the learner to use the vector macro.
It is easy to assume that the code needs to read from `a` which is not
what is intended.
The updated excercise introduces the syntax for initial array and vector
contents and lets the learner figure out how to tweak initial values of
a vector. This learning seems to be the original intent of the
excercise.
