private Function getPreviousStartFunction(Navigatable navigatable) {
			ProgramLocation location = getProgramLocation(navigatable);
			if (location == null) {
				return null;
			}

			Address currentAddress = location.getAddress();
			Program program = location.getProgram();
			FunctionManager functionManager = program.getFunctionManager();
			return functionManager.getFunctionContaining(currentAddress);
		}
Find the previous history LocationMemento that contains a different function. If no such
LocationMemento is found, null is returned.

@param navigatable the navigatable being navigated
@param moveTo true means after finding, get current location to it. false to just find
           and do nothing
@return previous LocationMemento, or null if no previous function found
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/navigation/NavigationHistoryPlugin.java