Preferred declarative approach for outputting tallies

I have a general question for those who adhere to declarative methods.
What is your preferred regime for outputting results of analyzing a
sequence of items? The "analysis" here might be complicated, requiring
state variables, counters, and triggers (to determine when the
counting occurs). Then the "results" would be the final values of
those counters.

The main issue I'm thinking about is determining when the sequence has
been consumed so you know when to write the outputs to the result
document.

I simply use the <xsl:iterate> instruction, and then the
<xsl:on-completion> instruction takes care of this question of
determining when the sequence is finished.