-The search conditions take the form of a key which is a single letter,
-followed by a space and a regexp, which can itself contain spaces. If
-the condition is prefaced by the character "!" it is negated, and has
+A search condition is either a time-related condition
+('today', 'yesterday', etc.), or a single-character key, followed by a
+space and a regexp, which can itself contain spaces.
+
+If it is prefaced by the character "!", a condition is negated and has