EXTERNAL

FUNCTION rtflsp (dum, x1, x2, xacc)

    DECLARE FUNCTION func         ! Supplied by the user

    LET maxit = 30
    LET fl = func(x1)
    LET fh = func(x2)
    IF fl * fh > 0 then
       PRINT "root must be bracketed for false position."
       EXIT FUNCTION
    END IF
    IF fl < 0 then
       LET xl = x1
       LET xh = x2
    ELSE
       LET xl = x2
       LET xh = x1
       LET swap = fl
       LET fl = fh
       LET fh = swap
    END IF
    LET dx = xh - xl
    FOR j = 1 to maxit
        LET trtflsp = xl + dx * fl / (fl - fh)
        LET f = func(trtflsp)
        IF f < 0 then
           LET del = xl - trtflsp
           LET xl = trtflsp
           LET fl = f
        ELSE
           LET del = xh - trtflsp
           LET xh = trtflsp
           LET fh = f
        END IF
        LET dx = xh - xl
        LET rtflsp = trtflsp
        IF abs(del) < xacc or f = 0 then EXIT FUNCTION
    NEXT j
    PRINT "rtflsp exceed maximum iterations"

END FUNCTION