Function operator

src/math-json/utils.ts:134–144

(
  expr: MathJsonExpression | null | undefined
)

 * * `["Negate", 5]`  -> `"Negate"`
 */
export function operator(
  expr: MathJsonExpression | null | undefined
): MathJsonSymbol {
  if (Array.isArray(expr)) return expr[0];

  if (expr === null || expr === undefined) return '';

  if (isFunctionObject(expr)) return expr.fn[0];

  return '';
}

/**
 * Return the arguments of a function, or an empty array if not a function