These functions extract a designated subset of a hyperframe,
  or replace the designated subset with another hyperframe.
The function [.hyperframe is a method for the subset operator
  [ for the
  class "hyperframe". It extracts the subset of x
  specified by the row index i and column index j.
The argument drop determines whether the array structure
  will be discarded if possible. The argument strip determines
  whether the list structure in a row or column or cell will be discarded if
  possible.
  If drop=FALSE (the default), the return value is always
  a hyperframe or data frame.
  If drop=TRUE, and if the selected subset has only one row,
  or only one column, or both, then
The function [<-.hyperframe is a method for the
  subset replacement operator [<- for the
  class "hyperframe". It replaces the designated
  subset with the hyperframe value.
  The subset of x to be replaced is designated by
  the arguments i and j as above.
  The replacement value should be a hyperframe with the
  appropriate dimensions, or (if the specified subset is a single
  column) a list of the appropriate length.
The function $.hyperframe is a method for $
  for hyperframes. It extracts the relevant column of the hyperframe.
  The result is always a list (i.e. equivalent to using
  [.hyperframe with strip=FALSE).
The function $<-.hyperframe is a method for $<-
  for hyperframes. It replaces the relevant column of the hyperframe.
  The replacement value should be a list of the appropriate length.