code: function toVersionString(versions) {
    return Object.keys(versions).sort().filter(key => versions[key]).map((key) => {
      return versions[key] ? `${key}@${versions[key]}` : '';
    }).join(', ');
  }
docs: /*
  Turns a POJO with versions into a string of the following format:
  
  key@value, key@value, key@value.
  
  Ignores `null` or `undefined` values and sorts keys alphabetically.
  
  @return {String}