Extensions to the FOR iteration macro

We have also made two extensions to the for iteration construct: a keyed-by clause and in ... using clauses.

The keyed-by clause allows binding a variable to the collection key associated with each element. This is particularly useful for iteration over tables or other explicit key collections:

for (v keyed-by k in collection)
  ...
end;

The in ... using clause allows you to specify a iteration protocol other than the default (forward-iteration-protocol):

for (element in my-sequence using backward-iteration-protocol)
  ...
end;