Calculating Tempi for midi output

[live action]

I was wondering if the metronome number in a \tempo command could be a scheme number.  This would make it easier to do effects for fermata and ritards and relative tempo changes, in midi files.

For example, it would be nice to be able to do something like this:

----------------
#(define base-tempo 100)

\tempo 4 = #base-tempo

% some music
% insert markup for "half note = dotted quarter" metronome mark
% and override so the following is invisible in the score

\tempo 4 = #( / ( * base-tempo 3) 4)
---------------

This way, one could change the base tempo in one place, and all the tempi would adjust automatically.

I'm guessing from the error messages I get when I try this, that  this is a feature request, but it would be awesome to be able to do this.

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