verbatimtex
\documentclass[11pt]{article}
\usepackage{times}
\begin{document} etex
.PS
  arrow "$u$" above
S: circle rad 10/72.27  # 10 pt
  line right 0.35
G: box "$G(s)$"
  arrow "$y$" above
  line down G.ht from last arrow then left last arrow.c.x-S.x then to S.s
  "$-\;$" below rjust
  arrow <- down 0.1 from S.s
.PE
end
