Just as awk has FILENAME NR FNR like variables Add same feature in sed.Now sed has "F" for filename "=" for line numberBut flaw in sed is that those F and = directky ouyput to stdout.
Using debug i found above
Rather if F and = result can be made a part of pattern space we can perform many one liners wuthout mulyiple piped commands.Eg.
I want 4th line of file prepended with filenzme
currently we have to sed '4F' | some more sed command to process to merge Filename line to 4th line.

But if F and = output can be made availabk in pattern space.eg 1F would add Filename\n1st line in partten space.
or 
sed '4s/^/$F/'
Juat like awk built in variable we canuse F and = options

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