R help - Aug 2003 - delete variable

Why in the codigo that continues when is erased the menubutton herself is 
not erased tambien the variable one opc, though itself deberia you erase 
with the assign?  
library(tcltk)
vectPath<-c()
opc<-tclVar(init=" ")

archivos<-function(){
          f<-tkcmd("tk_getOpenFile")
          temparch<-tclvalue(f)
          assign("vectPath",c(vectPath,temparch),.GlobalEnv)
          cant<-length(vectPath)
          j<-vectPath[cant]         
          tkadd(m, "radio", label=j, variable="opc",
value=j)
}

ver<-function(){ 
print(tclvalue("opc"))
}

borra<-function(){
	 assign("vectPath",c(),.GlobalEnv)
	 assign("opc"," ",.GlobalEnv)
     tkdelete(m,"0","end")
}
tt <- tktoplevel()
tkpack(mb <- tkmenubutton(tt, text="Datos"))
m <- tkmenu(mb,tearoff=FALSE) 
tkconfigure(mb,menu=m)
b<-tkbutton(tt,text="abrir",command=function()archivos())
tkpack(b)
b3<-tkbutton(tt,text="ver",command=function()ver())
tkpack(b3)
b4<-tkbutton(tt,text="Borrar",command=function()borra())
tkpack(b4)