parser: improve template hierarchy and error messages

[abiosoft]

No description provided.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 96.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93.25%. Comparing base (66e8643) to head (4b4cdb9).

Files with missing lines	Patch %	Lines
engine.go	94.91%	2 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
+ Coverage   91.20%   93.25%   +2.04%     
==========================================
  Files           3        3              
  Lines         307      341      +34     
==========================================
+ Hits          280      318      +38     
+ Misses         17       14       -3     
+ Partials       10        9       -1     

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