#everything but the last x lines:
x=10;seq -s $'\n' 1 30 | sed  -n ':s;1,'$((x-1))'{N;bs};{N;P;D}'

#everything but the first y lines and the last x lines:
y=5;x=10;seq -s $'\n' 1 30 | sed  -n '1,'$y'd;:s;1,'$((y+x-1))'{N;bs};{N;P;D}'

#stop at the 6th line matching foo:
 sed  '/foo/{H;x;s/\(\n.*\)\{6\}//;tq;x;b;:q;q}'

#join the lines of paragraph separated by a blank line:
 sed ':s;{N;s/\n\(.\{1,\}\)/ \1/;ts;s/\n//}' 

# grep -o '"[^"]"' 

sed -n $'s/[^"]*"\\([^"]*\\)"/\\1\\\n/;te;D;:e;P;D'
sed $'s/[^"]*"\\([^"]*\\)"/\\1\\\n/;te\nD;:e\nP;D'

#change the newline into ', '
sed '1!s/^/ /' | paste -s -d ','
sed  ':s;$!{N;s/\n/, /;bs}'

#print then n th block delimited by ## 
sed -n '/##/{x;s/a\{'$n'4\}/&/;tb;s/^/a/;x;b;:b;x;:a;p;n;/##/!ba;p;q}

tree(1):
<twkm> find ./ -type d -print | sed -e 's;[^/]*/;|____;g;s;____|;    |;g

#remove all occurence of foo...the unreadable one:
echo "foo bar foo" | sed  ':;;\;foo;s;;;;t;'

#tac
sed 'x;1!H;$!d;g' file 
(oneliners version: sed '1!G;h;$!d')