introduce Prolog text in engine and refine directives

[ichiban]

No description provided.

[codecov]

Codecov Report

Base: 98.05% // Head: 98.20% // Increases project coverage by +0.15% 🎉

Coverage data is based on head (a5ef5ca) compared to base (afd9006).
Patch coverage: 98.59% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #257      +/-   ##
==========================================
+ Coverage   98.05%   98.20%   +0.15%     
==========================================
  Files          21       22       +1     
  Lines        7137     7140       +3     
==========================================
+ Hits         6998     7012      +14     
+ Misses        109      102       -7     
+ Partials       30       26       -4     

Impacted Files	Coverage Δ
engine/dcg.go	96.10% <75.00%> (+0.21%)	⬆️
engine/parser.go	98.65% <94.73%> (+0.01%)	⬆️
engine/builtin.go	97.72% <100.00%> (+0.17%)	⬆️
engine/clause.go	98.63% <100.00%> (ø)
engine/text.go	100.00% <100.00%> (ø)
engine/vm.go	86.99% <100.00%> (+0.70%)	⬆️
interpreter.go	100.00% <100.00%> (+0.73%)	⬆️
... and 1 more

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