FUNCTION TelegraphProcess,t,lambda,p,X0=x0
;+
; S=TelegraphProcess(t,lamda,p)
; Generates a random process with X[t]=X[0] if
; the number of zero crossings in (0,t) is even and
; X[t]=-X[0] if the number is odd. The waveform is
; returned in an array of size n=lambda*t/p. If
; p is not specified then the default is p=0.1. The
; probability should be small enough that the Poisson
; assumptions are satisfied. The initial value X[0] can
; be specified with the keyword X0. If it is unspecified,
; it is randomly initialized to +1 or -1.
;
; Harvey Rhody
; March, 2000
;-
IF N_PARAMS() LT 3 THEN p=0.1
n=lambda*t/p
u=FIX(RANDOMN(SEED,n,BINOMIAL=[1,p])>0)
s=INTARR(n)
IF KEYWORD_SET(x0) THEN S[0]=x0 ELSE $
	IF RANDOMN(SEED) GE 0 THEN S[0]=1 ELSE S[0]=-1
FOR k=1,n-1 DO s[k]=s[k-1]*(1-2*u[k-1])
RETURN,s
END