readFormula = async formulaNumber => {
    const { file: formulaPath } = makeFormulaPath(formulaNumber);
    const formulaText = await filePowers.maybeReadFileText(formulaPath);
    if (formulaText === undefined) {
      throw new ReferenceError(`No reference exists at path ${formulaPath}`);
    }
    const formula = (() => {
      try {
        return JSON.parse(formulaText);
      } catch (error) {
        throw new TypeError(
          `Corrupt description for reference in file ${formulaPath}: ${error.message}`,
        );
      }
    })();
    return formula;
  }
@param {string} formulaNumber
@returns {Promise<Formula>}