not and unknown boolean logic gates are exported to SBGN-ML without a value set for the class attribute which results in invalid SBGN-ML
Although these logical operators do not exist in SBGN (the not operator exists but it is not binary as in CellDesigner), it seems they are exported as glyphs with no value for the class attribute in SBGN-ML, making the resulting file invalid. Probably they should be skipped?