protected abstract ValueType mergeValues(FrameType frame, int slot, ValueType a, ValueType b) throws DataflowAnalysisException;
protected abstract void mergeValues(FrameType otherFrame, FrameType resultFrame, int slot) throws DataflowAnalysisException;
	protected abstract ValueType mergeValues(FrameType frame, int slot, ValueType a, ValueType b)	        throws DataflowAnalysisException;