How to align filled output line with tab stop?

[Lucas de Sena]

Consider the following troff source file (There is a tab character after
each "Left" string (The ^A is actually a Ctrl+A character (Sorry about
the long line.)))

        .ds L This is a very long string that will not fit in a single output 
line and will have to break
        .po 1i
        .ll 5i
        .ta 1i 4i
        .sp 1i
        .fi
        Left    Middle^ARight
        .sp 1v
        Left    \*L^ARight

It outputs a document like this:

        Left     Middle..............................................Right
        Left     This is a very long string that will not fit in a single output
        line and will have to breakRight

It sets two tab stops: one at 1 inch from the left and the other at
4 inches from the left.

It then outputs a line with the content "Left" in the beginning of the
output line, "Middle" at the first tab stop, and "Right" at the second
tab stop.

It does the same with the second input line, but this time, the "Middle"
content is very long, and, since we are in fill mode (.fi), it is broken
down into two output lines.

However, I want the output to be like this:  I want the "Right" content
of the second input line to be aligned with the second tab stop.  I also
want the long "Middle" content of the second input line to be filled
between the two tab stops.  Like this:

        Left     Middle..............................................Right
        Left     This is a very long string that will not fit in a
                 single output line and will have to break...........Right

I played a lot with \l and \h to obtain such behavior, but with no
success.

How can I obtain such behavior?

I'm using Heirloom Doctools, but I want the solution to be portable, if
possible.

Regards,
Seninha