Introduction In comparison with the embodiment using special heddles, this embodiment contains no further components such as half-heddles for mediating the positive contact with the warp threads and whose motion shows discontinuous changes during the heddle frame alternations that by their impacts have adverse effects on the operation frequence of the weaving machine. However, the drawback of the mechanism described in the patent CZ 280643 consists in that it permits to produce the leno weave only with a limited number of warp threads on the fabric edge, and not a complete leno fabric. Another drawback of the described embodiment consists in that it fails to ensure the same tension in each of the two warp thread systems, and consequently, in the uneven proportionate elongation during the weaving process. For this reason, warp threads to be interlaced in this way must be supplied from special accessory warp thread bobbins, each of them equipped with an independently adjustable brake. Another well-known device for producing the leno weave on the fabric edge is described in EP 152 956 A2 and EP 450 120 A1, intended for double gripper looms for weaving double fabrics. For producing the edge on each fabric, it comprises one system of stationary threads, and two systems of laterally deflectable movable threads. The movable threads are led across oblique grooves responsible for their movement like in the preceding solution, and the stationary threads are led from supplementary warp bobbins into the eyes of the vertically movable threads while passing through a guide aperture provided in the needle holder situated outside the longitudinal axis of the needle so that the rotating threads are spliced/interconnected with each other only in the front shed, i.e., in the section between the beat-up point and the needle's eye, but are permanently separated from each other in the section between the guide aperture in the needle holder and the warp bobbin. After passing through the guide aperture, the stationary threads make a turn of 90 DEG and pass along the needles into the needle eyes from the needle front side to the needle rear side after which they again arrive at the needle front side. The drawback of this solution consists in particular in great strain imposed on the stationary threads so that the device can be used only for the creation of the fabric edges where the warp threads are supplied from special supplementary warp bobbins with individual adjustment of the thread tensions or with the thread tension adjustment carried out separately for the stationary threads on the one hand and for the movable warp threads on the other hand. Disclosure of the invention The above drawbacks of the state of art have been eliminated by the device for controlling warp threads according to this invention whose principle consists in that at least two compensation rollers of the rotary warp thread system are mounted on a third heddle frame, the guide member of the system of the stationary threads being situated in front of the plane passing through the system of needles fitted with eyes, considered in the direction of the travel of the warp threads. The device permits to keep equal traction forces both in the system of stationary warp threads and in the system of rotating warp threads and to eliminate the differences of their proportionate elongation in the area extending from the beat-up point to the whip roll during the weaving cycle and weaving process on a weaving machine and permits a continuous passage of stationary warp threads without sharp bends. Preferably, the compensation rollers of the rotating warp thread system, mounted on the third heddle frame, are arranged adjustably with respect to this third heddle frame, thus permitting to form and adjust the required angle of contact of the rotating strand of the warp threads for producing the frictional resistance whose values are then continuously identical with the resistance of the warp threads in the stationary strand. Also preferably, the compensation rollers of the rotating warp thread system are mounted on a swinging shaft, adapted reversingly to swing on the third heddle frame (and coupled with the drive mechanism). This permits easily and continuously to change the angle of contact of the rotating warp thread system on the compensation rollers and by means of this, to modify the frictional force acting on the warp threads of the rotating warp thread system. Preferably, the guide member of the stationary warp thread system is made as a longitudinal bar parallel with the heddle frames. To obtain the optimum function of the mechanism, the guide member is preferably mounted on the connecting rods of the mechanism used to bring about the reversible motion of the system of needles fitted with eyes. The advantage of this embodiment consists in the fact that the needle stroke governing the motion of the stationary warp thread system passing over the guide member is approximately the same as the stroke of the oblique guide slots governing the motion of the rotating warp thread system. Each warp thread belonging to the stationary warp thread system passes through the needle's eye carrying out a substantially rectilinear reversible motion during the shed formation. After the warp thread system has left the needle, these warp threads pass via the guide member situated on the connecting rod of the drive mechanism. The trajectory of the guiding of this system has been chosen so as to ensure that the total length of the warp thread belonging to the stationary warp remains constant in the whole section between the beat-up point and the whip roll during the weaving cycle. Having passed through the oblique guide slots provided in the heddle frame, the warp threads belonging to the rotating warp thread system pass through a compensation loop consisting of a pair of compensation rollers situated preferably as near as possible the heddle frame equipped with oblique slots. The equal tension of the two warp strands in the area of the fabric formation ensures equal weaving ability and fabric quality. Moreover, the elimination of the difference in the proportionate elongation through the section from the harness to the whip roll permits to make use of only one warp beam and only one whip roll. Brief description of the drawings