List environments are environments that have list-like properties.
For instance, the elements of a list environment are ordered and
can be accessed and iterated over using index subsetting,
 e.g.
 'x <- listenv(a = 1, b = 2);
  for (i in seq_along(x)) x[[i]] <- x[[i]] ^ 2;
  y <- as.list(x)'.

WWW: https://cran.r-project.org/web/packages/listenv/