compute the average of the values v1 to v6 after removing the smallest and
largest.
+B<AVG>
+
+Pop one element (I<count>) from the stack. Now pop I<count> elements and build the
+average, ignoring all UNKNOWN values in the process.
+
+Example: C<CDEF:x=a,b,c,d,4,AVG>
+
B<TREND>
Create a "sliding window" average of another data series.