product rule

echo f\' = $(cat <<< ghl | \awk '{i=length()+1; while(i-1) print( substr($0,0,i-1) "'\''" substr($0,i--) )}' |\awk 'BEGIN {RS = ":"; FS="\n"} {i= NF-2; print $(NF-1); while(i) {printf(" + %s", $i ); i--}; print "\n" }')f' = g'hl + gh'l + ghl'