Well, sorry but sed _is_ cryptic. Let me quote an exemple for you (squeezing blank lines):

>leaves a blank line at the beginning and end if there are some already.

     #!/usr/bin/sed -f

     # on empty lines, join with next 
     # Note there is a star in the regexp 
     :x 
     /^\n*$/ { 
       N
       bx
     }
     
     # now, squeeze all '\n', this can be also done by:
     # s/^\(\n\)*/\1/
     s/\n*/\
     /

As soon as you begin to use sed registers, the code becomes arcanic.