MODULE rkdumb

    PUBLIC xx(0), y(0,0)          ! COMMON, for keeping values

    SUB rkdumb (vstart(), nvar, x1, x2, nstep, dum)

        LIBRARY "rk4"

        ! The subroutine derivs is supplied by the user

        DIM v(0), dv(0)
        MAT redim v(nvar), dv(nvar)

        FOR i = 1 to nvar
            LET v(i) = vstart(i)
            LET y(i, 1) = v(i)
        NEXT i
        LET xx(1) = x1
        LET x = x1
        LET h = (x2 - x1) / nstep
        FOR k = 1 to nstep

            CALL derivs (x, v(), dv())
            CALL rk4 (v(), dv(), nvar, x, h, v(), dum)

            IF x + h = x then
               PRINT "Stepsize not significant in rkdumb."
               EXIT SUB
            END IF

            LET x = x + h
            LET xx(k + 1) = x
            FOR i = 1 to nvar
                LET y(i, k + 1) = v(i)
            NEXT i
        NEXT k

    END SUB

END MODULE