[igraph] igraph class functions

[Peter McMahan]

Hi,
I noticed an inconsistency in the way that igraph functions handle  
classes. Most functions check the input graph with a call to is.igraph:
function (graph)
{
    res <- (class(graph) == "igraph")
    res
}

The problem is that in R, class inheritence is handled by classes of  
length greater than 1. So if I'm trying to make a new class called  
"mygraph" that extends "igraph", then I'll set my class attribute to  
be c("mygraph","igraph"). Then if I call plot() it will look for  
plot.mygraph() and if it doesn't find it will revert to plot.igraph 
(). With the current class catching, this will then thow a warning  
and an error.

May i suggest the following change?:

is.igraph <- function(graph){
    res <- ("igraph" %in% class(graph))
    res
}

With this definition objects for which "igraph" is not the first  
class will still be passable to igraph-specific functions.

Thanks,
Peter McMahan