4.26. How do I join two lines if line #2 begins in a [certain string]?

The inverse situation is another FAQ. Suppose a line begins with a particular string. How do you bring that line up to follow the previous line? In this example, we want to match the string "<<=" at the beginning of one line, bring that line up to the end of the line before it, and replace the string with a single space:

     sed -e :a -e '$!N;s/\n<<=/ /;ta' -e 'P;D' file   # all seds
     sed ':a; $!N;s/\n<<=/ /;ta;P;D' file             # GNU, ssed, sed15+