Fix a bug with rule _&R

[winitzki]

Trying to use curryhoward for live demo in chapter 4 lead to a failure. This PR fixes the associated bugs.

[codecov]

Codecov Report

Merging #7 into master will decrease coverage by 0.11%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
- Coverage   92.09%   91.98%   -0.12%     
==========================================
  Files           6        6              
  Lines         468      474       +6     
  Branches       23       28       +5     
==========================================
+ Hits          431      436       +5     
- Misses         37       38       +1

Impacted Files	Coverage Δ
src/main/scala/io/chymyst/ch/Macros.scala	100% <ø> (ø)	⬆️
src/main/scala/io/chymyst/ch/LJT.scala	97.94% <100%> (-0.02%)	⬇️
src/main/scala/io/chymyst/ch/TheoremProver.scala	100% <100%> (ø)	⬆️
src/main/scala/io/chymyst/ch/TermExpr.scala	87.43% <96.29%> (ø)	⬆️

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