Macro BOLDing 2+ lines

I am trying to debug one my macro and finally I reduced it to the simplest form: it accepts an argument, wraps it with "set-bold", "reset-bold" font requests and that's all. But the problem is that I want to pass it multiple lines as 1 argument. I tried to embed "\n" inside the string, but without success (I still don't know if it's possible). So, I switched to a diversion:

.\" Start a diversion to capture text                                                                                         
.di t1                                                                                                                        
aaa                                                                                                                           
                                                                                                                              
bbb                                                                                                                           
                                                                                                                              
.di                                                                                                                           
                                                                                                                              
.\" Define a macro to print the diversion content in bold                                                                     
.de MMM                                                                                                                      
.ft B                                                                                                                         
\\$1                                                                                                                          
.                                                                                                                             
.ft R                                                                                                                         
..                                                                                                                            
                                                                                                                              
.\" Use the macro to process and print the diversion (I am
.\" not sure about asciify too)
.asciify t1                                                                                                                   
.MMM \*[t1]

and it prints

aaa

bbb

I.e. "bbb" is not bold! It was a problem to force it to print "bbb", not only "aaa" (I tried .br between lines but it seems only empty line works fine). And after this, I hit impossibility to bold ALL lines (in the case they are only 2).

So how to force it to make "bbb" bold, I have not idea. I don't understand even why it happens. Usually such trick works but not with multi-line arguments.

How to fix this simple macro?

You cannot give multiple lines as an argument to a macro call.

When you do the call to MMM, actually only the first line is given as argument. To show this:

.\" Start a diversion to capture text                                                                                         
.di t1
aaa

bbb

.di

.\" Define a macro to print the diversion content in bold                                                                     
.de MMM
.ft B
\\$1
.
.ft R
.br
MMM end
..

.\" Use the macro to process and print the diversion (I am
.\" not sure about asciify too)
.asciify t1                                                                                                                   
.MMM \*[t1]

which gives:

So the bbb is never seen by the macro MMM. The actual input for groff would be

.MMM aaa

bbb

There are two ways to gt around this problem. First: use a known diversion name. If you always use t1 as the diversion name, simply do

.de MMM2
.ft B
\*[t1]
.ft R
..

Or use the name of the diversion as argument:

.de MMM3
.ft B
\\*[\\$1]
.ft R
..

Some explanation about diversions: a diversion is used to divert the formatted text into a named storage area. What you need is unformatted text. .asciify basically unformats the diversion again.

Putting it all together in one demo, you'll get:

.\" Start a diversion to capture text  
.di t1
aaa

bbb

.di

.di t2
ccc

ddd

.di

.\" Define a macro to print the diversion content in bold                                                                     
.de MMM
.ft B
\\$1
.
.ft R
.br
MMM end
..

.\" Unformat the diversions using asciify
.asciify t1                                                                                                                   
.asciify t2    

.\" Use the macro to process and print the diversion 
                                                                                                               
.MMM \*[t1]

.\" Use a known diversion name instead of an argument to the macro
.de MMM2
.ft B
\*[t1]
.ft R
..

.MMM2
.\" Use the name of the diversion as argument to the macro
.de MMM3
.ft B
\\*[\\$1]
.ft R
..

.MMM3 t1

.MMM3 t2

.MMM3 t1

with the output: