[Rd] slots of type "double"

[hpages at fhcrc.org]

Hi,

Any idea why S4 doesn't allow slots of type "double"?

  > setClass("A", representation(a="double"))
  Error in makePrototypeFromClassDef(properties, ClassDef, immediate, where) : 
    in making the prototype for class "A" elements of the prototype failed to
match the corresponding slot class: a (class “double” )

"numeric", "integer", "character", "complex", "raw", etc... they all work
but "double" doesn't. Any reason for this strange exception?

Thanks!
H.