1.

'%-%' <- function(x,y) max(x-y,0)

2.

globalSort <- function(xname) 
{
   tmp <- sort(get(xname,envir=.GlobalEnv))
   assign(xname,tmp,envir=.GlobalEnv)
}

3.

dfsort <- function(df,col) 
{
   if (is.numeric(col)) {
      toSort <- df[,col]  
   } else toSort <- df[[col]]
   # or replace above by df[[col]]
   tmp <- order(toSort)
   df[tmp,]
}