;************************************************************************
PRO hyman, f, t, dt
if n_params(0) eq 0 then begin
  print,'PRO hyman, f, t, dt'
  return
endif
;
;  Hymans 3rd order leap frog time advance
;
;************************************************************************
  COMMON hymanc, fold, first ; Used by this routine itself /Bo.

  default,first,0.
  if first eq 0.0 then begin
    fnew = f + dt*pde(f)
    first=1.
  end else begin
    pdef = pde(f)
    fpredict=fold + 2.*dt*pdef
    fnew = 0.8*f + 0.2*fold + dt*(0.8*pdef + 0.4*pde(fpredict))
    first=first+1.
  end 
  ;
  fold = f
  f    = fnew
  t    = t + dt
END