markup circle and sub

[Gianmaria Lari]

This code works well and create a rehearsal mark in a box

\version "2.19.80"

\mark \markup {\box {A1}}

Now I would like the same except for the number that should be in subscript. What I should do? I tried this but does not work (it writes A and 1 in two different boxes):

\version "2.19.80"

\mark \markup {\box {A \sub 1}}

Thank you, g.