1 2all: 3 @for x in "" A= A=42; do ${.MAKE} -f ${MAKEFILE} show $$x; done 4 5show: 6 @echo "The answer is ${A:?known:unknown}" 7 @echo "The answer is ${A:?$A:unknown}" 8 @echo "The answer is ${empty(A):?empty:$A}" 9