modified(_, nomod) {
      const fields = this.fields;
      return !((nomod || this.mod.length) && fields) ? false : !arguments.length ? !!fields : isArray(_) ? _.some(f => fields[f]) : fields[_];
    }
Checks if one or more data fields have been modified during this pulse
propagation timestamp.
@param {string|Array<string>} _ - The field(s) to check for modified.
@param {boolean} nomod - If true, will check the modified flag even if
  no mod tuples exist. If false (default), mod tuples must be present.
@return {boolean} - Returns true if any of the provided fields has been
  marked as modified, false otherwise.