##A triangle class
##ideas from S. Keene, Object Oriented Programming in Common Lisp
#library(methods)

 setClass("triangle", representation(a="numeric", b="numeric",
       c="numeric"))

 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)

#what should: angleA(tr1)+angleB(tr1)+ angleC(tr1)
#be equal to?