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Many of iterate's clauses accumulate values into a variable, or set a
variable under certain conditions. At the end of the loop, this
variable contains the desired result. All these clauses have an
optional into keyword, whose argument should be a symbol. If
the into keyword is not supplied, the accumulation variable
will be internally generated and its value will be returned at the end
of the loop; if a variable is specified, that variable is used for the
accumulation, and is not returned as a result—it is up to the user
to return it explicitly, in the loop's epilogue code (see
finally). It is safe to examine the accumulation variable
during the loop, but it should not be modified.
These clauses all begin with a verb. When the verb does not conflict
with an existing Common Lisp function, then it may be used in either
its infinitival or present-participle form (e.g. sum,
summing). However, when there is a conflict with Common Lisp,
only the present-participle form may be used (e.g. unioning).
This is to prevent iterate clauses from clashing with Common Lisp
functions.