EXTERNAL

FUNCTION rtsec (dum, x1, x2, xacc)

    DECLARE FUNCTION func         ! Supplied by the user

    LET maxit = 30
    LET fl = func(x1)
    LET f = func(x2)
    IF abs(fl) < abs(f) then
       LET trtsec = x1
       LET xl = x2
       LET swap = fl
       LET fl = f
       LET f = swap
    ELSE
       LET xl = x1
       LET trtsec = x2
    END IF
    FOR j = 1 to maxit
        LET dx = (xl - trtsec) * f / (f - fl)
        LET xl = trtsec
        LET fl = f
        LET trtsec = trtsec + dx
        LET f = func(trtsec)
        LET rtsec = trtsec
        IF abs(dx) < xacc or f = 0 then EXIT FUNCTION
    NEXT j
    PRINT "rtsec exceed maximum iterations"

END FUNCTION