Add rightif(predicate, focus)

[tkf]

No description provided.

[codecov]

Codecov Report

Merging #32 into master will decrease coverage by 2.78%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
- Coverage   68.96%   66.17%   -2.79%     
==========================================
  Files           3        4       +1     
  Lines          58       68      +10     
==========================================
+ Hits           40       45       +5     
- Misses         18       23       +5     

Flag	Coverage Δ
#unittests	66.17% <50.00%> (-2.79%)	⬇️

Impacted Files	Coverage Δ
src/utils.jl	0.00% <0.00%> (ø)
src/reductions.jl	75.00% <62.50%> (-12.50%)	⬇️

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