Description

A block begins with a BEGIN line and ends with a END. The job is prepending a COMMENTS: line before the END line if the block does not have one.

Raw Input

Desired Output

some other
BEGIN
 block 1, line 1
 block 1, line 2
COMMENTS: add more
END
some other
BEGIN
 block 2, line 1
 block 2, line 2
END

some other
BEGIN
 block 1, line 1
 block 1, line 2
COMMENTS: add more
END
some other
BEGIN
 block 2, line 1
 block 2, line 2
COMMENTS:
END

Script and Comments

Script₁ [ 1] /^BEGIN$/!b [ 2] :loop [ 3] N [ 4] /\nEND$/!b loop [ 5] /\nCOMMENTS:/!s/\nEND$/\nCOMMENTS:&/
Comments Step [1] is used to skip lines between two blocks. When the first line of a block is reached, the loop consists of Steps [2] thru [4] is used to join all lines of the block to the Pattern Space. If a block does not have a COMMENTS:... line, Step [5] will insert it.