text: calculist.register('lmSessionStorage', ['_'], function (_) {
  
    var allowedKeys = ['zoomGuid','focusGuid'];
  
    var throwIfNotAllowed = function (key) {
      if(!_.includes(allowedKeys, key)) throw new Error('"' + key + '" is not a valid lmSessionStorage key.');
      return key;
    };
  
    return Object.freeze({
      set: function (key, val) {
        sessionStorage[throwIfNotAllowed(key)] = '' + val;
      },
      get: _.flow(throwIfNotAllowed, _.propertyOf(sessionStorage))
    });
  });
  