##written to get the string of the system call rather than having the
 ##system call actually executed. Initial motivation was for unit
 ##testing.
 asSystemCall <- function(x) {
   expr <- substitute(x)
   options(systemCallMode = TRUE)
   on.exit(options(systemCallMode = FALSE))
   error <- tryCatch(eval(expr, parent.frame()),
                          error = function(e) return(e))
   error$systemCall
 }