Scheme function returning number of measures in a system

[Mike Solomon]

Hey lilypond-users,
    I am working on a scheme function that will automate the setting of
TupletNumber #'text with respect to the amount of measures in a given
system.  Currently, this minimal example prints the 8 whole notes for each
system, whereas I would like it to print 2, 2, 2, 2 for the four systems
respectively (each of which consists of 2 measures).

\version "2.13.0"

\relative c'' { << {
    \once \override Voice . TupletNumber #'text = #(markup #:note "1" UP "+"
#:note "1" UP "+" #:note "1" UP "+" #:note "1" UP "+" #:note "1" UP "+"
#:note "1" UP "+" #:note "1" UP "+" #:note "1" UP)
    \times 1/1 { \repeat unfold 8 { d1 }  } } \\ { \repeat unfold 32 { a16 [
b c b ] } } >> 
}


If there exists a scheme function I could call that would give me the # of
measures in a system (or, alternatively, if TupletNumber has a
parent/grandparent/great-grandparent... that contains this info), I can
tweak the TupletNumber after-line-break callback function.

THANK YOU!
~Mike