.PS
# . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . #
gen_init
  linewid = linewid*0.8
  circlerad = 0.35/2
  bw = boxwid/2
  bh = boxht/2

Multiblock: [
boxht = 0.4
boxwid = boxht
circlerad = boxht*0.4

define  summer { [ C: circle
  line from C.nw to C.se
  line from C.sw to C.ne
  "$$1$" at C.n+(0,1bp__) below
  "$$2$" at C.s-(0,1bp__) above
  "$$3$" at C.e+(1bp__,0) rjust
  "$$4$" at C.w-(1bp__,0) ljust ]
}
define doublebox {[box "$$1$"; arrow; box "$$2$"]}

  arrow "$E_1$" above
S1: summer(,-,,+)
  linewid = boxwid/2
  arrow "$\epsilon$" above
  doublebox(H_1,H_2)
  arrow
S2: summer(-,,,+)
  arrow
T: doublebox(H_3,H_4)
  line
Y: Here
  arrow
  "$S_1$" above
R1: box "$R_1$" at (T.x,T.y+boxht*5/4)
  arrow from Y to (Y,R1) then to R1.e
  arrow from R1.w to (S2,R1) then to S2.n
  "$n_1$" ljust at S2.n+(0,boxht/2)
  left
Rf: doublebox(R_2,R_3) at ((S1.x+T.x)/2,T.y-boxht*5/4)
  arrow from T to (T,Rf) then to Rf.e
  arrow from Rf.w to (S1,Rf) then to S1.s
  "$n_2$" at S1.s+(0,-boxht/2) ljust
] #with .nw at Observer.sw + (0,-0.3)
# "A multiblock example" below ljust at last [].sw+(0,-3pt__)
.PE