code: function(prefix, f) {
      var retF = function(env) {
          Object.keys(env)
              .forEach(function(x) {
                  var val = env[x];
                  if ((typeof val === 'string') &&
                      (val.indexOf(prefix) === 0)) {
                      var propName = val.substring(prefix.length,
                                                   val.length);
                      env[x] = f(propName);
                  } else if (Array.isArray(val)) {
                      retF(val);
                  } else if (val && (typeof val === 'object')) {
                      retF(val);
                  }
              });
      };
      return retF;
  }
docs: Returns a function that filters relevant values in an environment and
  applies a transform to them.
  
  @param {string} prefix A matching prefix for selected values.
  @param {function(string): Object} f A function that transforms matching
  values.