EXTERNAL

FUNCTION rtbis (dum, x1, x2, xacc)

    DECLARE FUNCTION func         ! Supplied by the user

    LET jmax = 40
    LET fmid = func(x2)
    LET f = func(x1)
    IF f * fmid >= 0 then
       PRINT "root must be bracketed for bisection."
       EXIT FUNCTION
    END IF

    IF f < 0 then
       LET trtbis = x1
       LET dx = x2 - x1
    ELSE
       LET trtbis = x2
       LET dx = x1 - x2
    END IF
    FOR j = 1 to jmax
        LET dx = dx * .5
        LET xmid = trtbis + dx
        LET fmid = func(xmid)
        IF fmid <= 0 then LET trtbis = xmid
        LET rtbis = trtbis
        IF abs(dx) < xacc or fmid = 0 then EXIT FUNCTION
    NEXT j
    PRINT "too many bisections"

END FUNCTION