added eval-when form so %create-concept-symbol is available to macros. Also wrapped +UNSET-IMPLEMENTATION-TAG+ to be bound only once even if compiled/lodaed/executed
added eval-when form so %create-concept-symbol is available to macros. Also wrapped +UNSET-IMPLEMENTATION-TAG+ to be bound only once even if compiled/lodaed/executed

velezj/j-plankton