These functions extract a designated subset of an object of class
  "fv", or replace the designated subset with other data,
  or delete the designated subset.
The subset is specified by the 
  row index i and column index j, or
  by the column name name. Either i or j
  may be missing, or both may be missing.
The function [.fv is a method for the generic operator
  [ for the class "fv". It extracts the
  designated subset of x, and returns it as
  another object of class "fv" (if drop=FALSE)
  or as a data frame or vector (if drop=TRUE).
The function [<-.fv is a method for the generic operator
  [<- for the class "fv".
  If value is NULL, the designated subset of x will be
  deleted from x.
  Otherwise, the designated subset of x will be
  replaced by the data contained in value.
  The return value is the modified object x.
The function $<-.fv is a method for the generic operator
  $<- for the class "fv".
  If value is NULL, the designated column of x will be
  deleted from x.
  Otherwise, the designated column of x will be
  replaced by the data contained in value.
  The return value is the modified object x.