docs: function(event, opt_lookForEndEvent) {
        var sf;
        var stack;
        if (opt_lookForEndEvent) {
          sf = event.esf;
          stack = event.estack;
        } else {
          sf = event.sf;
          stack = event.stack;
        }
        if (stack !== undefined && sf !== undefined) {
          this.model_.importWarning({
            type: 'stack_frame_and_stack_error',
            message: 'Event at ' + event.ts +
                ' cannot have both a stack and a stackframe.'
          });
          return undefined;
        }
  
        if (stack !== undefined)
          return this.model_.resolveStackToStackFrame_(event.pid, stack);
        if (sf === undefined)
          return undefined;
  
        var stackFrame = this.model_.stackFrames['g' + sf];
        if (stackFrame === undefined) {
          this.model_.importWarning({
            type: 'sample_import_error',
            message: 'No frame for ' + sf
          });
          return;
        }
        return stackFrame;
      }