Function builtin_solve 

fn builtin_solve(
    args: &[RValue],
    named: &[(String, RValue)],
) -> Result<RValue, RError>

solve(a, b) — solve linear system or compute matrix inverse via LU decomposition.

• solve(a): returns the inverse of matrix a
• solve(a, b): solves the linear system Ax = b

Uses nalgebra’s LU decomposition with partial pivoting for numerical stability.