document another sed portability problem

[Bruno Haible]

Hi,

Another sed portability problem hit me:

sed -e 'a\
text'

should insert the entire text. But the native sed on MacOS X 10.3 and
10.4 omits the initial whitespace at the beginning of the text line.
Can we document this?

Bruno

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  On the other hand, no white space is allowed between @samp{:} and the
  subsequent label name.
  
+ The @samp{a} command requires a backslash-newline before the text to be
+ added.  Furthermore, if this text starts with whitespace, you need to
+ insert a backslash before it, at the beginning of the line.  This is
+ because otherwise the @command{sed} program on MacOS X 10.4 eats up this
+ whitespace:
+ 
+ @example
+ $ @kbd{echo foo | sed -e 'a\}
+ @kbd{        bar'}
+ foo
+ bar
+ $ @kbd{echo foo | sed -e 'a\}
+ @kbd{\        bar'}
+ foo
+         bar
+ @end example
+ 
  @item @command{sed} (@samp{t})
  @c ---------------------------
  @prindex @command{sed} (@samp{t})