function makeRows(m, n) {
    let row = "<th></th>";
    for (let j = 1; j <= n; j++) 
        row += "<th>"+j+"</th>";
    let text = "<tr>"+row+"</tr>";
    for (let i = 1; i <= m; i++) {
       row = "<th>"+i+"</th>";
       for (let j = 1; j <= n; j++) {
          let p = n*(i-1)+j;
          let s = "<span class=tip>C"+p+"</span>"
          row += "<td>"+i+","+j+s+"</td>";
       }
       text += "<tr>"+row+"</tr>";
    }
    let t = text.length+" chars";
    console.log("makeRows: "+m+"x"+n+" "+t);
    tablo.innerHTML = text; out.innerText = t; 
}