@Override
    boolean execute() {
		message = "";

		// get the start for what will be the start of a
		// tempory block
		length = endAddr.subtract(block.getEnd());
		if (length == 0) {
			return true;
		}
		try {
			startAddr = endAddr.subtract(length-1); 
			return expandBlock();
		} catch(Exception e) {
			message = e.getMessage();
			if (message  == null) {
				message = e.toString();
			}
		}
		return false;
	}
@see ghidra.app.plugin.core.memory.ExpandBlockModel#execute()
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/memory/ExpandBlockDownModel.java