renamed copy for exception's underlying term

[ichiban]

#192

[codecov-commenter]

Codecov Report

Merging #194 (ae8b7da) into main (002821b) will decrease coverage by 0.03%.
The diff coverage is 95.49%.

@@            Coverage Diff             @@
##             main     #194      +/-   ##
==========================================
- Coverage   96.22%   96.18%   -0.04%     
==========================================
  Files          20       20              
  Lines        6250     6243       -7     
==========================================
- Hits         6014     6005       -9     
- Misses        196      198       +2     
  Partials       40       40              

Impacted Files	Coverage Δ
engine/builtin.go	94.41% <91.71%> (-0.01%)	⬇️
engine/exception.go	99.00% <98.55%> (-1.00%)	⬇️
engine/clause.go	97.82% <100.00%> (ø)
engine/iterator.go	100.00% <100.00%> (ø)
engine/number.go	100.00% <100.00%> (ø)
engine/stream.go	100.00% <100.00%> (ø)
engine/vm.go	82.74% <100.00%> (ø)
interpreter.go	94.88% <100.00%> (ø)

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