setClass("shape", representation( area = "numeric") )

 setClass("triangle", representation("shape", a = "numeric",
                b="numeric", c="numeric") )
 tr1 <- new("triangle", a=3,b=4,c=5)

 setClass("rectangle", representation("shape", a = "numeric",
                                      b="numeric") )
  rtgl1 <- new("rectangle", a=3,b=4)

  setClass("square", representation("shape", a = "numeric") )
  sq1 <- new("square", a=3)

  setClass("lgtdl", representation(times="numeric", values="numeric"))

  lgtdl1 <- new("lgtdl", times=1:10, values=rnorm(10))
  lgtdl2 <- new("lgtdl", times=1:10, values=runif(10))

  setMethod("Arith", signature(e1="lgtdl", e2="lgtdl"), 
   function(e1, e2) {
       if( !identical(e1@times, e2@times) )
          stop("can only add lgtdl's with identical times")
       e1@values = callGeneric(e1@values, e2@values)
       e1
    })


  ##old triangle code

 tr1 <- new("triangle", a=3, b=4, c=5)

 ##Law of cosines: a^2 = b^2+c^2-2*b*c*cos(A)
 threesidestoangle <- function(a=a, b=b, c=c)
    acos((b^2+c^2-a^2)/(2*b*c))

 angleA <- function(object)
    threesidestoangle(a=object@a,b=object@b, c=object@c)

 angleB <- function(object)
    threesidestoangle(object@b, object@c, object@a)

 angleC <- function(object)
    threesidestoangle(object@c, object@a, object@b)

 if( ! isGeneric("area") )
    setGeneric("area", function(object) standardGeneric("area"))

 setMethod("area", signature(object="triangle"),
           function(object)
               object@a*object@b*sin(angleC(object))/2 )

 area(tr1)