Some minor touchup to preconditions and fix for minus.

[haz]

Proposed changes

Forgo negating an expression for the ability to subtract two expressions.

Fixes

Fixes #100, but also breaks the use of - as something that negates.

Types of changes

What types of changes does your code introduce?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Further comments

More discussion over in #100

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.76%. Comparing base (502c7a4) to head (caf21a1).
⚠️ Report is 105 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
+ Coverage   88.35%   88.76%   +0.41%     
==========================================
  Files          25       25              
  Lines        1794     1798       +4     
  Branches      333      335       +2     
==========================================
+ Hits         1585     1596      +11     
+ Misses        149      144       -5     
+ Partials       60       58       -2     

Flag	Coverage Δ
unittests	88.76% <100.00%> (+0.41%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pddl/parser/domain.py	78.36% <100.00%> (+2.10%)	⬆️

... and 1 file with indirect coverage changes

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[haz]

Tests all run green, but marking this as WIP until we resolve the discussion over in #100

[haz]

I do plan on getting back to this at some point...