Convert set to sealed interface.

[vincentk]

Convert set to sealed interface permitting non-empty and empty subtypes only.

[github-actions]

Overall Project	65.47% -2.11%	🍏
Files changed	41.94%	❌

File	Coverage
PoSet.java	100%	🍏
EmptySet.java	100%	🍏
Set.java	83.33%	🍏
NonEmptySet.java	67.86% -32.14%	🍏
Magma.java	50%	🍏
Union.java	0% -75%	❌

[codecov]

Codecov Report

Attention: Patch coverage is 61.53846% with 5 lines in your changes missing coverage. Please review.

Project coverage is 60.13%. Comparing base (e950f1a) to head (865fbe0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../java/com/github/vincentk/dedekind/sets/Union.java	33.33%	2 Missing and 2 partials ⚠️
...com/github/vincentk/dedekind/sets/NonEmptySet.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #80      +/-   ##
============================================
+ Coverage     59.64%   60.13%   +0.49%     
- Complexity       54       59       +5     
============================================
  Files            40       40              
  Lines           280      286       +6     
  Branches         40       42       +2     
============================================
+ Hits            167      172       +5     
+ Misses           93       92       -1     
- Partials         20       22       +2     

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[github-actions]

Overall Project	66.63% -1.74%	🍏
Files changed	63.41%	🍏

File	Coverage
PoSet.java	100%	🍏
EmptySet.java	100%	🍏
SingletonSet.java	95.83%	🍏
NonEmptySet.java	93.1% -6.9%	🍏
Set.java	79.63%	🍏
Magma.java	50%	🍏
Union.java	44% -56%	❌

[github-actions]

Overall Project	66.63% -1.74%	🍏
Files changed	63.41%	🍏

File	Coverage
PoSet.java	100%	🍏
EmptySet.java	100%	🍏
SingletonSet.java	95.83%	🍏
NonEmptySet.java	93.1% -6.9%	🍏
Set.java	79.63%	🍏
Magma.java	50%	🍏
Union.java	44% -56%	❌

[github-actions]

Overall Project	67.5% -0.87%	🍏
Files changed	81.71%	🍏

File	Coverage
PoSet.java	100%	🍏
EmptySet.java	100%	🍏
SingletonSet.java	95.83%	🍏
NonEmptySet.java	93.1% -6.9%	🍏
Set.java	79.63%	🍏
Union.java	74% -26%	🍏
Magma.java	50%	🍏

[github-actions]

Overall Project	67.5% -0.87%	🍏
Files changed	81.71%	🍏

File	Coverage
PoSet.java	100%	🍏
EmptySet.java	100%	🍏
SingletonSet.java	95.83%	🍏
NonEmptySet.java	93.1% -6.9%	🍏
Set.java	79.63%	🍏
Union.java	74% -26%	🍏
Magma.java	50%	🍏
DirectedSet.java	0%	🍏
SemiLattice.java	0%	🍏