Suggested by Jeremy Stribling.
<mutator> must be "insert" or "delete".
If <mutator> is "insert", then each of the key-value pairs
- in the map in <value> is added to <column> if its key is
- not already present. The required type of <value> is
+ in the map in <value> is added to <column> only if its key
+ is not already present. The required type of <value> is
slightly relaxed, in that it may have fewer than the
minimum number of elements specified by the column's type.