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 iteration over table elements:

for (my-element keyed-by my-key in my-table)
  ...
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;