function seleccionar_todo(form){

	   for (i=0;i<document.getElementById(form).length;i++)
	      if(document.getElementById(form).elements[i].type == "checkbox")
	    	  document.getElementById(form).elements[i].checked=1;
	} 


function deseleccionar_todo(form){
	   for (i=0;i<document.getElementById(form).length;i++)
		      if(document.getElementById(form).elements[i].type == "checkbox")
		    	  document.getElementById(form).elements[i].checked=0;
		}

function todos(selTodas,variable,totsel){
	  var size=document.getElementById(variable).length;
	  if (selTodas) document.getElementById(variable).value=+size;
	  else document.getElementById(totsel).value=0;
	  for (var i = 0; i < size ; i++)
		  document.getElementById(variable).options[i].selected=selTodas;
	}