Wallaby currently supports special parameter value-assignments that begin with distinguished prefixes.  When these are applied to a working configuration, modify an existing value, if it exists, rather than replacing it.  For example, ">= VALUE" appends ", VALUE" to the preexisting parameter.  This bug suggests adding a "?=" prefix that will conditionally assign a value to a parameter only if it does not already have a value.  So a feature that gives PARAM a value of "?= VALUE" will have no effect when applied to a working configuration that already contains a value for PARAM, but it will assign "VALUE" to PARAM if the working configuration does not have a value for PARAM.