step. This allows you to do calculations across the data. This
function cannot be used in B<VDEF> instructions.
+B<PREV(vname)>
+
+Pushes an I<unknown> value if this is the first value of a data
+set or otherwise the result of vname variable at the previous time
+step. This allows you to do calculations across the data. This
+function cannot be used in B<VDEF> instructions.
+
Z<>
=item Time