MAKEFLOW_INPUTS=input
MAKEFLOW_OUTPUTS=mydir mydir/1.txt mydir/3.txt

# second rule should not have to create 'mydir' in its command
mydir mydir/1.txt: input
	mkdir -p mydir; cp input/hello mydir/1.txt; cp input/hello mydir/2.txt

mydir/3.txt: mydir/1.txt
	echo hello > mydir/3.txt; cat mydir/1.txt > mydir/3.txt