docs: function group() {                              // experimental
     var o,i=0,a=arguments; 
     if(a[0].length) a = a[0]; 
     
     if((typeof(a[i]) == "object") && (a[i] instanceof CAG)) {
        o = a[i].extrude({offset: [0,0,0.1]});    // -- convert a 2D shape to a thin solid, note: do not a[i] = a[i].extrude()
     } else {
        o = a[i++];                               
     }
     for(; i<a.length; i++) { 
        var obj = a[i];
        if((typeof(a[i]) == "object") && (a[i] instanceof CAG)) {
           obj = a[i].extrude({offset: [0,0,0.1]});    // -- convert a 2D shape to a thin solid:
        }
        o = o.unionForNonIntersecting(obj); 
     } 
     return o; 
  }