reverseMergeSubstitutions(substitutions) {
    if (typeof substitutions === 'undefined' || substitutions === null) {
      return;
    }
    if (typeof substitutions !== 'object') {
      throw new Error(
        'Object expected for `substitutions` in reverseMergeSubstitutions'
      );
    }
    this.substitutions = Object.assign({}, substitutions, this.substitutions);
  }
Reverse merge substitutions, preserving existing ones