Define substitute, split off generic code

[mtfishman]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 67.59777% with 58 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.13%. Comparing base (0c19c8b) to head (c007956).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lazynameddimsarrays.jl	67.59%	58 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   48.76%   54.13%   +5.37%     
==========================================
  Files           4        4              
  Lines         365      447      +82     
==========================================
+ Hits          178      242      +64     
- Misses        187      205      +18     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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