[Fix] Making SubstituteAccordingTo relies on BOTH the variable ID AND the environment

[kwesiRutledge]

Noticed an interesting bug when we are transforming optimization problems: The substitutions needed to represent one problem's variables in terms of another's started to fail when the corresponding variables had matching ID values EVEN if they were from different problems.

This method attempts to address that by making SubstituteAccordingTo does replacement based on BOTH:

• ID
• Environment where the variable comes from.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.11%. Comparing base (e2bac28) to head (0f2a040).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
symbolic/basic_environment.go	66.66%	5 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   87.00%   87.11%   +0.10%     
==========================================
  Files          42       43       +1     
  Lines        6394     6416      +22     
==========================================
+ Hits         5563     5589      +26     
+ Misses        734      727       -7     
- Partials       97      100       +3     

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[Copilot]

Pull Request Overview

Fix substitution to respect both variable ID and its originating environment to avoid collisions when variables from different problems share IDs.

• Introduces an Environment interface and a BasicEnvironment implementation with a DefaultEnvironment.
• Extends Variable with an Environment reference and updates constructors and vector/matrix helpers to work with environments.
• Adds tests for SubstituteAccordingTo to ensure environment-aware substitution and an example using vector constraints.

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
symbolic/environment.go	Replaces concrete Environment struct with an interface.
symbolic/basic_environment.go	Adds BasicEnvironment implementation, MakeBasicEnvironment, and DefaultEnvironment.
symbolic/variable.go	Adds Environment field to Variable, updates variable constructors to track env, and changes substitution to consider environment.
symbolic/variable_vector.go	Updates function signature to accept Environment interface and uses DefaultEnvironment.
symbolic/variable_matrix.go	Updates signatures to accept Environment interface and uses DefaultEnvironment.
testing/symbolic/utils_test.go	Updates tests to construct Variables with an Environment.
testing/symbolic/variable_test.go	Updates env creation in tests to use MakeBasicEnvironment.
testing/symbolic/polynomial_test.go	Adds new test verifying environment-aware substitution.
examples/constraints1/vector_constraints.go	New example demonstrating vector constraints.

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[Copilot]

Assigning a pointer to a local interface variable (&currentEnv) embeds a pointer-to-interface whose address differs across constructor calls, breaking intuitive equality of Variable values from the same environment. Once the Environment field is changed to Environment (non-pointer), set this to: Environment: currentEnv.

[Copilot]

Same issue as above for NewBinaryVariable: avoid storing &currentEnv (pointer to interface). After changing the field type to Environment, assign the interface value directly: Environment: currentEnv.

[Copilot]

Changing Environment from a concrete struct to an interface is a breaking API change. Consider adding a migration path: e.g., provide a type alias for the old struct (if applicable), document the change, and add deprecated wrapper constructors/functions that accept *BasicEnvironment to ease transition, or add helper constructors like NewBasicEnvironmentPtr() returning *BasicEnvironment for callers that previously passed *Environment.