LaTeX parsing: Invalid subscript with complex number `i` (e.g. `i_A`)

[tejashah88]

Description

It seems that i with any subscript is not being processed properly, very similar to #255. The compute engine still tries to treat i as a complex number, instead of treating i_A as its own variable.

Steps to Reproduce

import { ComputeEngine } from "@cortex-js/compute-engine";

const ce = new ComputeEngine();

const BATCH_1 = [
    'i',
    'i+1',
    'i_A',
    'i_A+1',
    '\frac{i_{A}}{i}',
];

function convertLatexToMathJson(latex) {
    const parsedExpr = ce.parse(latex);
    const boxedExpr = ce.box(parsedExpr, { canonical: true });
    return boxedExpr.json;
}

// Run testing
console.log('Batch 1 Testing');
for (const expr of BATCH_1) {
    const mjson = convertLatexToMathJson(expr);
    console.log(expr, ' => ', JSON.stringify(mjson, null, 2));
}

Actual Result

Batch 1 Testing
i  =>  [
  "Complex",
  0,
  1
]
i+1  =>  [
  "Add",
  1,
  [
    "Complex",
    0,
    1
  ]
]
i_A  =>  [
  "Subscript",
  [
    "Complex",
    0,
    1
  ],
  "A"
]
i_A+1  =>  [
  "Add",
  [
    "Error",
    [
      "ErrorCode",
      "'incompatible-type'",
      "number",
      "expression"
    ]
  ],
  1
]
\frac{i_{A}}{i}  =>  [
  "Divide",
  [
    "Error",
    [
      "ErrorCode",
      "'incompatible-type'",
      "number",
      "expression"
    ]
  ],
  [
    "Complex",
    0,
    1
  ]
]

Environment

Testing code is ran against Node.js v22.20.0 and NPM v10.9.3 using compute engine v0.30.2.