Function expression

src/math-json/parse-sympy.ts:355–362

(
  s: string,
  i: number
)

}

function expression(
  s: string,
  i: number
): [index: number, result: Expression | null] {
  let lhs: Expression | null;
  [i, lhs] = primary(s, i);
  return [i, lhs];
}

function parenth_form(
  s: string,