3.6 Truncating Aggregations  When looking at aggregation
results, you often care only about the top several results. The
keys and values associated with anything other than the highest
values are not interesting. You might also wish to discard an
entire aggregation result, removing both keys and values. The
DTrace trunc function is used for both of these situations.  The
parameters to trunc are an aggregation and an optional
truncation value. Without the truncation value, trunc discards
both aggregation values and aggregation keys for the entire
aggregation. When a truncation value n is present, trunc
discards aggregation values and keys except for those values and
keys associated with the highest n values. That is, trunc(@foo,
10) truncates the aggregation named foo after the top ten
values, where trunc(@foo) discards the entire aggregation. The
entire aggregation is also discarded if 0 is specified as the
truncation value.  To see the bottom n values instead of the top
n, specify a negative truncation value