P_KEYS[tx + (4 * ty)]
      }
      const start = e => {
        for(let z=0; z<e.touches.length; z++) {
          const i = e.touches[z].identifier
          const k = pointToKey(e.touches[z])
          if (k != null) down(k)
          tmap[i]=k
        }
        pd(e)
      }
      const move = e => {
        for(let z=0; z<e.touches.length; z++) {
          const i = e.touches[z].identifier
          const k = pointToKey(e.touches[z])
          if (tmap[i] == k) continue       // same cell, nothing to do