add unary plus

[ichiban]

#144

[codecov-commenter]

Codecov Report

Merging #157 (2932af9) into main (3f3c314) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #157   +/-   ##
=======================================
  Coverage   93.99%   93.99%           
=======================================
  Files          18       18           
  Lines        5360     5361    +1     
=======================================
+ Hits         5038     5039    +1     
  Misses        272      272           
  Partials       50       50           

Impacted Files	Coverage Δ
engine/builtin.go	91.39% <100.00%> (+<0.01%)	⬆️
interpreter.go	92.61% <100.00%> (ø)

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