+h4(#filterexpression). Filtering by comparing two attributes
+
+If @a@ and @b@ are attributes that have numeric values, they can be compared in a filter condition using the form @["(a < b)", "=", true]@. Supported operators are @=@, @<@, @<=@, @>@, and @>=@. Examples:
+* @["(replication_desired < replication_confirmed)", "=", true]@
+* @["(replication_desired = replication_confirmed)", "=", true]@
+
+Note that only a very specific subset of boolean expressions is supported. For example:
+* @["replication_desired < replication_confirmed", "=", true]@ is not accepted. Outer parentheses are mandatory.
+* @["(replication_desired < 1)", "=", true]@ is not accepted. Operands must be attribute names.
+