## noheader

Check Indecies of an Expression


  # PARSE THIS: 
  for (i in 1:N)
    X[i+k] <- 
    X[(1:k) + (i-1)] * thet[1:k], epsi[(1:j) + (i-1) + (k-j)] * phi[1:j]



  # INTO THIS:
  inds <- data.frame(i=integer(), X=character(), Xprev=character(), thet=character(), S=character(), epsi=character(), phi=character())
  for (i in 1:N)
    inds <- rbind(inds, cbind(
                      i     =  i, 
                      X     =  paste_(i+k + xshft), 
                      Xprev =  paste_((1:k) + (i-1) + xshft), 
                      thet  =  paste_(1:k), 
                      S     =  " ", 
                      epsi  =  paste_((1:j) + (i-1) + eshft ), 
                      phi   =  paste_(1:j)
                  ))

  inds <- data.table(inds)
  inds[c(1:6, (N-12):nrow(inds)), ]





  x <- 1:max(as.numeric(as.character(inds$X)))
  toGrab <- ((N:1)+(length(x)-N))
  inds[inds$X %in% toGrab, ]