Crash after SymPy fails to parse variable named `is` in mod file

[kbvw]

When running NMODL on a mod file (kv4_ch.mod, found in https://github.com/Hjorthmedh/BasalGangliaData/tree/main/data/neurons/mechanisms) containing a variable named is, SymPy throws a SyntaxError that later results in NMODL crashing with seemingly unrelated errors.

(.venv_nmodl) vanwalst@bbpv1 ~/p/snudda> nmodl BasalGangliaData/data/neurons/mechanisms/kv4_ch.mod
INFO : Using nmodl-nightly Package (Developer Version)
[NMODL] [warning] :: SympySolverVisitor :: solve_non_lin_system python exception: Sympify of expression 'could not parse 'is'' failed, because of exception being raised:
SyntaxError: invalid syntax (<string>, line 1)
[NMODL] [warning] :: SympySolverVisitor :: solve_non_lin_system python exception: Sympify of expression 'could not parse 'is'' failed, because of exception being raised:
SyntaxError: invalid syntax (<string>, line 1)
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
[NMODL] [error] :: NeuronSolveVisitor :: solver method 'sparse' not supported
terminate called after throwing an instance of 'std::runtime_error'
  what():  PRIME encountered during code generation, ODEs not solved?
fish: Job 1, 'nmodl BasalGangliaData/data/neu…' terminated by signal SIGABRT (Abort)

After renaming the variable, NMODL runs without error. Tested on nightly 0.6.152 and on 0.5.