A function value table (object of class "fv")
  is a convenient way of storing and plotting
  several different estimates of the same function.
  Objects of this class are returned 
  by many commands in spatstat, such as Kest
  which returns an estimate of Ripley's $K$-function
  for a point pattern dataset.  Sometimes it is useful to convert the function value table
  to a function in the Rlanguage. This is done by
  as.function.fv. It converts an object x of class "fv"
  to an Rfunction f.
  If f <- as.function(x) then f is a function with one numeric
  argument, that performs linear interpolation between the values
  in the table x. Argument values lying outside the range of the
  table yield an NA value.
  The command as.function.fv is a method for the generic command
  as.function.