Derive used vars from group expression

[DeLaGuardo]

Fix #68

[codecov]

Codecov Report

Merging #69 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage    95.7%   95.71%   +<.01%     
==========================================
  Files          19       19              
  Lines        1561     1563       +2     
  Branches       52       52              
==========================================
+ Hits         1494     1496       +2     
  Misses         15       15              
  Partials       52       52

Impacted Files	Coverage Δ
src/axel_f/analyzer.cljc	100% <100%> (ø)	⬆️

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