[S02] relax Duration a bit

Durations are now dimensional but not restrictive.  Added some
speculation about how to approach dimensional analysis gradually.

S02-bits.pod

@@ -13,8 +13,8 @@ Synopsis 2: Bits and Pieces
 
     Created: 10 Aug 2004
 
-    Last Modified: 16 Nov 2010
-    Version: 228
+    Last Modified: 17 Nov 2010
+    Version: 229
 
 This document summarizes Apocalypse 2, which covers small-scale
 lexical items and typological issues.  (These Synopses also contain
@@ -1355,14 +1355,16 @@ be taken as durations, as above); you may not add two instants.
 
     $instant + $instant      # WRONG
     $instant - $instant      # ok, returns a duration
-    $instant + $duration     # ok, returnas an instant
-
-Durations allow additive operations with other durations, and allow
-any numeric operation with a number as the other argument:
-
-    $duration * $duration    # WRONG, durations aren't geometric
-    $duration * 2            # ok, a duration twice as long
-    2 * $duration            # same
+    $instant + $duration     # ok, returns an instant
+
+Numeric operations on durations return C<Duration> where that makes
+sense (addition, subtraction, modulus).  The type returned for other
+numeric operations is unspecified; they may return normal numeric
+types or they may return other dimensional types that attempt to
+assist in dimensional analysis.  (The latter approach should likely
+require explicit declaration for now, until we can demonstrate that
+it does not adversely impact the average programmer, and that it
+plays well with the concept of gradual typing.)
 
 The C<Blob> type is like an immutable buffer, and therefore
 responds both to array and (some) stringy operations.  Note that,