code: function(F) {
          var validSet = [];
          for (var i = 0; i < F.length; i++) {
              if (F[i].isValid()) {
                  validSet.push(F[i]);
              }
          }
          return validSet;
      }
docs: Returns valid facts using explicit facts' validity tags.
  @param F
  @param refs
  @returns {Array}